virtio: fix virtio_config_ops description

- get_features has returned 64 bits since commit d0254773
  ("virtio: add support for 64 bit features.")
- properly mark all optional callbacks

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>