virtio: add VIRTIO_RING_NO_LEGACY

Add macro to disable legacy vring functions.

Signed-off-by: Matej Genci <matej.genci@nutanix.com>
Link: https://lore.kernel.org/r/20190911124942.243713-1-matej.genci@nutanix.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>