dirty-bitmap: rename bdrv_undo_clear_dirty_bitmap

Use more generic names to reuse the function for bitmap merge in the
following commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 92ecbd8..f605622 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -1155,7 +1155,7 @@
 void bdrv_set_dirty(BlockDriverState *bs, int64_t offset, int64_t bytes);
 
 void bdrv_clear_dirty_bitmap(BdrvDirtyBitmap *bitmap, HBitmap **out);
-void bdrv_undo_clear_dirty_bitmap(BdrvDirtyBitmap *bitmap, HBitmap *in);
+void bdrv_restore_dirty_bitmap(BdrvDirtyBitmap *bitmap, HBitmap *backup);
 
 void bdrv_inc_in_flight(BlockDriverState *bs);
 void bdrv_dec_in_flight(BlockDriverState *bs);