qom: Use atomics for object refcounting

Object reference counts will soon be changed outside the BQL. So we need
to use atomics in object_ref/unref.

Based on a patch by Liu Ping Fan.

Signed-off-by: Liu Ping Fan <qemulist@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qom/object.c

@@ -683,16 +683,15 @@ GSList *object_class_get_list(const char *implements_type,
 
 void object_ref(Object *obj)
 {
-    obj->ref++;
+     atomic_inc(&obj->ref);
 }
 
 void object_unref(Object *obj)
 {
     g_assert(obj->ref > 0);
-    obj->ref--;
 
     /* parent always holds a reference to its children */
-    if (obj->ref == 0) {
+    if (atomic_fetch_dec(&obj->ref) == 1) {
         object_finalize(obj);
     }
 }