virtio: unexport virtio_finalize_features

virtio_finalize_features is only used internally within virtio.
No reason to export it.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>