migration: forbid bitmap migration by generated node-name

It actually never worked with libvirt, as auto-generated names are
different on source and destination.

It's unsafe and useless to migrate by auto-generated node-names, so
let's forbid it.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: NAndrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20200521220648.3255-7-vsementsov@virtuozzo.com>
[eblake: shorter subject line]
Signed-off-by: NEric Blake <eblake@redhat.com>

migration/block-dirty-bitmap.c

@@ -285,6 +285,13 @@ static int add_bitmaps_to_list(BlockDriverState *bs, const char *bs_name)
         return -1;
     }
 
+    if (bs_name[0] == '#') {
+        error_report("Bitmap '%s' in a node with auto-generated "
+                     "name '%s' can't be migrated",
+                     bdrv_dirty_bitmap_name(bitmap), bs_name);
+        return -1;
+    }
+
     FOR_EACH_DIRTY_BITMAP(bs, bitmap) {
         if (!bdrv_dirty_bitmap_name(bitmap)) {
             continue;