blockdev: fix drive_backup_prepare() missed error

We leak local_err and don't report failure to the caller. It's
definitely wrong, let's fix.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20210202124956.63146-5-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

blockdev.c

@@ -1825,9 +1825,7 @@ static void drive_backup_prepare(BlkActionState *common, Error **errp)
     aio_context_acquire(aio_context);
 
     if (set_backing_hd) {
-        bdrv_set_backing_hd(target_bs, source, &local_err);
-        if (local_err) {
-            error_propagate(errp, local_err);
+        if (bdrv_set_backing_hd(target_bs, source, errp) < 0) {
             goto unref;
         }
     }