virtio: fix virtio_config_ops description

- get_features has returned 64 bits since commit d025477368792
  ("virtio: add support for 64 bit features.")
- properly mark all optional callbacks

Signed-off-by: Cornelia Huck <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Halil Pasic <[email protected]>
1 file changed