block: remove bdrv_truncate callback in blkdebug

Now that bdrv_truncate is passed to bs->file by default, remove the
callback from block/blkdebug.c and set is_filter to true. is_filter also gives
access to other callbacks that are forwarded automatically to bs->file for
filters.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Manos Pitsidianakis <el13635@mail.ntua.gr>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/blkdebug.c

@@ -808,12 +808,6 @@ static int64_t blkdebug_getlength(BlockDriverState *bs)
     return bdrv_getlength(bs->file->bs);
 }
 
-static int blkdebug_truncate(BlockDriverState *bs, int64_t offset,
-                             PreallocMode prealloc, Error **errp)
-{
-    return bdrv_truncate(bs->file, offset, prealloc, errp);
-}
-
 static void blkdebug_refresh_filename(BlockDriverState *bs, QDict *options)
 {
     BDRVBlkdebugState *s = bs->opaque;
@@ -896,6 +890,7 @@ static BlockDriver bdrv_blkdebug = {
     .format_name            = "blkdebug",
     .protocol_name          = "blkdebug",
     .instance_size          = sizeof(BDRVBlkdebugState),
+    .is_filter              = true,
 
     .bdrv_parse_filename    = blkdebug_parse_filename,
     .bdrv_file_open         = blkdebug_open,
@@ -904,7 +899,6 @@ static BlockDriver bdrv_blkdebug = {
     .bdrv_child_perm        = bdrv_filter_default_perms,
 
     .bdrv_getlength         = blkdebug_getlength,
-    .bdrv_truncate          = blkdebug_truncate,
     .bdrv_refresh_filename  = blkdebug_refresh_filename,
     .bdrv_refresh_limits    = blkdebug_refresh_limits,