qapi: fix memory leak in bdrv_image_info_specific_dump

The 'obj' result of the visitor was not properly freed, like done in
other places doing a similar job.
Signed-off-by: NPino Toscano <ptoscano@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qapi.c

@@ -698,6 +698,7 @@ void bdrv_image_info_specific_dump(fprintf_function func_fprintf, void *f,
     assert(qobject_type(obj) == QTYPE_QDICT);
     data = qdict_get(qobject_to_qdict(obj), "data");
     dump_qobject(func_fprintf, f, 1, data);
+    qobject_decref(obj);
     visit_free(v);
 }