virtio: teach virtio_has_feature() about transport features

Drivers don't add transport features to their table, so we
shouldn't check these with virtio_check_driver_offered_feature().

We could perhaps add an ->offered_feature() virtio_config_op,
but that perhaps that would be overkill for a consitency check
like this.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>