block: use bdrv_add_before_write_notifier

Register the notifier using the specific API for block devices.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/write-threshold.c

@@ -76,8 +76,7 @@ static int coroutine_fn before_write_notify(NotifierWithReturn *notifier,
 static void write_threshold_register_notifier(BlockDriverState *bs)
 {
     bs->write_threshold_notifier.notify = before_write_notify;
-    notifier_with_return_list_add(&bs->before_write_notifiers,
-                                  &bs->write_threshold_notifier);
+    bdrv_add_before_write_notifier(bs, &bs->write_threshold_notifier);
 }
 
 static void write_threshold_update(BlockDriverState *bs,