blockjob: Update description of the 'device' field in the QMP API

The 'device' field in all BLOCK_JOB_* events and 'block-job-*' command
is no longer the device name, but the ID of the job. This patch
updates the documentation to clarify that.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

docs/qmp-events.txt

@@ -92,7 +92,8 @@ Data:
 
 - "type":     Job type (json-string; "stream" for image streaming
                                      "commit" for block commit)
-- "device":   Device name (json-string)
+- "device":   Job identifier. Originally the device name but other
+              values are allowed since QEMU 2.7 (json-string)
 - "len":      Maximum progress value (json-int)
 - "offset":   Current progress value (json-int)
               On success this is equal to len.
@@ -116,7 +117,8 @@ Data:
 
 - "type":     Job type (json-string; "stream" for image streaming
                                      "commit" for block commit)
-- "device":   Device name (json-string)
+- "device":   Job identifier. Originally the device name but other
+              values are allowed since QEMU 2.7 (json-string)
 - "len":      Maximum progress value (json-int)
 - "offset":   Current progress value (json-int)
               On success this is equal to len.
@@ -143,7 +145,8 @@ Emitted when a block job encounters an error.
 
 Data:
 
-- "device": device name (json-string)
+- "device": Job identifier. Originally the device name but other
+            values are allowed since QEMU 2.7 (json-string)
 - "operation": I/O operation (json-string, "read" or "write")
 - "action": action that has been taken, it's one of the following (json-string):
     "ignore": error has been ignored, the job may fail later
@@ -167,7 +170,8 @@ Data:
 
 - "type":     Job type (json-string; "stream" for image streaming
                                      "commit" for block commit)
-- "device":   Device name (json-string)
+- "device":   Job identifier. Originally the device name but other
+              values are allowed since QEMU 2.7 (json-string)
 - "len":      Maximum progress value (json-int)
 - "offset":   Current progress value (json-int)
               On success this is equal to len.

qapi/block-core.json

@@ -713,7 +713,8 @@
 #
 # @type: the job type ('stream' for image streaming)
 #
-# @device: the block device name
+# @device: The job identifier. Originally the device name but other
+#          values are allowed since QEMU 2.7
 #
 # @len: the maximum progress value
 #
@@ -1475,7 +1476,9 @@
 #
 # Throttling can be disabled by setting the speed to 0.
 #
-# @device: the device name
+# @device: The job identifier. This used to be a device name (hence
+#          the name of the parameter), but since QEMU 2.7 it can have
+#          other values.
 #
 # @speed:  the maximum speed, in bytes per second, or 0 for unlimited.
 #          Defaults to 0.
@@ -1506,7 +1509,9 @@
 # operation can be started at a later time to finish copying all data from the
 # backing file.
 #
-# @device: the device name
+# @device: The job identifier. This used to be a device name (hence
+#          the name of the parameter), but since QEMU 2.7 it can have
+#          other values.
 #
 # @force: #optional whether to allow cancellation of a paused job (default
 #         false).  Since 1.3.
@@ -1532,7 +1537,9 @@
 # the operation is actually paused.  Cancelling a paused job automatically
 # resumes it.
 #
-# @device: the device name
+# @device: The job identifier. This used to be a device name (hence
+#          the name of the parameter), but since QEMU 2.7 it can have
+#          other values.
 #
 # Returns: Nothing on success
 #          If no background operation is active on this device, DeviceNotActive
@@ -1552,7 +1559,9 @@
 #
 # This command also clears the error status of the job.
 #
-# @device: the device name
+# @device: The job identifier. This used to be a device name (hence
+#          the name of the parameter), but since QEMU 2.7 it can have
+#          other values.
 #
 # Returns: Nothing on success
 #          If no background operation is active on this device, DeviceNotActive
@@ -1578,7 +1587,9 @@
 #
 # A cancelled or paused job cannot be completed.
 #
-# @device: the device name
+# @device: The job identifier. This used to be a device name (hence
+#          the name of the parameter), but since QEMU 2.7 it can have
+#          other values.
 #
 # Returns: Nothing on success
 #          If no background operation is active on this device, DeviceNotActive
@@ -2424,7 +2435,8 @@
 #
 # @type: job type
 #
-# @device: device name
+# @device: The job identifier. Originally the device name but other
+#          values are allowed since QEMU 2.7
 #
 # @len: maximum progress value
 #
@@ -2455,7 +2467,8 @@
 #
 # @type: job type
 #
-# @device: device name
+# @device: The job identifier. Originally the device name but other
+#          values are allowed since QEMU 2.7
 #
 # @len: maximum progress value
 #
@@ -2478,7 +2491,8 @@
 #
 # Emitted when a block job encounters an error
 #
-# @device: device name
+# @device: The job identifier. Originally the device name but other
+#          values are allowed since QEMU 2.7
 #
 # @operation: I/O operation
 #
@@ -2498,7 +2512,8 @@
 #
 # @type: job type
 #
-# @device: device name
+# @device: The job identifier. Originally the device name but other
+#          values are allowed since QEMU 2.7
 #
 # @len: maximum progress value
 #