qom: Remove parent pointer when unparenting

Certain parts of the QOM framework test this pointer to determine if
an object is parented. Nuke it when the object is unparented to allow
for reuse of an object after unparenting.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

qom/object.c

@@ -402,6 +402,7 @@ void object_unparent(Object *obj)
     }
     if (obj->parent) {
         object_property_del_child(obj->parent, obj, NULL);
+        obj->parent = NULL;
     }
     object_unref(obj);
 }