block: remove bdrv_truncate callback in blkdebug

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>