virtio: use VRingMemoryRegionCaches for avail and used rings

The virtio-net change is necessary because it uses virtqueue_fill
and virtqueue_flush instead of the more convenient virtqueue_push.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>