virtio: Don't expose legacy config features when VIRTIO_CONFIG_NO_LEGACY defined.

The VIRTIO_F_ANY_LAYOUT and VIRTIO_F_NOTIFY_ON_EMPTY features are pre-1.0
only.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Acked-by: Michael S. Tsirkin <mst@redhat.com>