tracetool: report error on foo() instead of foo(void)

C functions with no arguments must be declared foo(void) instead of
foo().  The tracetool argument list parser has never accepted an empty
argument list.  This patch adds a clear error message for this error
case.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180110202553.31889-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/tracetool/__init__.py

@@ -75,6 +75,8 @@ def build(arg_str):
         res = []
         for arg in arg_str.split(","):
             arg = arg.strip()
+            if not arg:
+                raise ValueError("Empty argument (did you forget to use 'void'?)")
             if arg == 'void':
                 continue