numa: set the memory backend "is_mapped" field

Commit 2aece63 "hostmem: detect host backend memory is being used properly"
added a way to know if a memory backend is busy or available for use. It
caused a slight regression if we pass the same backend to a NUMA node and
to a pc-dimm device:

-m 1G,slots=2,maxmem=2G \
-object memory-backend-ram,size=1G,id=mem-mem1 \
-device pc-dimm,id=dimm-mem1,memdev=mem-mem1 \
-numa node,nodeid=0,memdev=mem-mem1

Before commit 2aece63, this would cause QEMU to print an error message and
to exit gracefully:

qemu-system-ppc64: -device pc-dimm,id=dimm-mem1,memdev=mem-mem1:
    can't use already busy memdev: mem-mem1

Since commit 2aece63, QEMU hits an assertion in the memory code:

qemu-system-ppc64: memory.c:1934: memory_region_add_subregion_common:
    Assertion `!subregion->container' failed.
Aborted

This happens because pc-dimm devices don't use memory_region_is_mapped()
anymore and cannot guess the backend is already used by a NUMA node.

Let's revert to the previous behavior by turning the NUMA code to also
call host_memory_backend_set_mapped() when it uses a backend.

Fixes: 2aece63c8a
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <146891691503.15642.9817215371777203794.stgit@bahia.lan>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

numa.c

@@ -463,6 +463,7 @@ void memory_region_allocate_system_memory(MemoryRegion *mr, Object *owner,
             exit(1);
         }
 
+        host_memory_backend_set_mapped(backend, true);
         memory_region_add_subregion(mr, addr, seg);
         vmstate_register_ram_global(seg);
         addr += size;