Instead, the call should be passed to bs->file if it exists, to allow
filter drivers to support those methods without implementing them. This
commit makes `drv->is_filter = true` imply that these callbacks will be
forwarded to bs->file by default, so disabling support for these
functions must be done explicitly.
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>