vhost-user: use an enum helper for features mask The VHOST_USER_PROTOCOL_FEATURE_MASK will be automatically updated when adding new features to the enum. Signed-off-by: Thibaut Collet <thibaut.collet@6wind.com> [Adapted from mailing list discussion - Marc-André] Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Tested-by: Thibaut Collet <thibaut.collet@6wind.com>