Fix hanging user monitor when using balloon command

Arghh... Adding missing S-O-B

Hi Anthony.  I wonder if there was a problem when importing my async
command handler patchset.  Since the 'balloon' command completes
immediately, it must call the completion callback before returning.
That call was missing but is added by the patch below.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

monitor.c

@@ -2309,6 +2309,7 @@ static int do_info_balloon(Monitor *mon, MonitorCompletion cb, void *opaque)
         return -1;
     }
 
+    cb(opaque, NULL);
     return 0;
 }