block: Drop useless bdrv_new() call

bdrv_append_temp_snapshot() uses bdrv_new() to create an empty BDS
before invoking bdrv_open() on that BDS. This is probably a relict from
when it used to do some modifications on that empty BDS, but now that is
unnecessary, so we can just set bs_snapshot to NULL and let bdrv_open()
do the rest.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -1470,8 +1470,7 @@ static int bdrv_append_temp_snapshot(BlockDriverState *bs, int flags,
     qdict_put(snapshot_options, "driver",
               qstring_from_str("qcow2"));
 
-    bs_snapshot = bdrv_new();
-
+    bs_snapshot = NULL;
     ret = bdrv_open(&bs_snapshot, NULL, NULL, snapshot_options,
                     flags, &local_err);
     snapshot_options = NULL;