migration: add_bitmaps_to_list: check disk name once

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200521220648.3255-6-vsementsov@virtuozzo.com>
[eblake: shorter subject line]
Signed-off-by: Eric Blake <eblake@redhat.com>

migration/block-dirty-bitmap.c

@@ -274,17 +274,22 @@ static int add_bitmaps_to_list(BlockDriverState *bs, const char *bs_name)
     DirtyBitmapMigBitmapState *dbms;
     Error *local_err = NULL;
 
+    bitmap = bdrv_dirty_bitmap_first(bs);
+    if (!bitmap) {
+        return 0;
+    }
+
+    if (!bs_name || strcmp(bs_name, "") == 0) {
+        error_report("Bitmap '%s' in unnamed node can't be migrated",
+                     bdrv_dirty_bitmap_name(bitmap));
+        return -1;
+    }
+
     FOR_EACH_DIRTY_BITMAP(bs, bitmap) {
         if (!bdrv_dirty_bitmap_name(bitmap)) {
             continue;
         }
 
-        if (!bs_name || strcmp(bs_name, "") == 0) {
-            error_report("Found bitmap '%s' in unnamed node %p. It can't "
-                         "be migrated", bdrv_dirty_bitmap_name(bitmap), bs);
-            return -1;
-        }
-
         if (bdrv_dirty_bitmap_check(bitmap, BDRV_BITMAP_DEFAULT, &local_err)) {
             error_report_err(local_err);
             return -1;