vioblk_request
r = vioblk_request(vib, xfer, VIRTIO_BLK_T_IN);
r = vioblk_request(vib, xfer, VIRTIO_BLK_T_OUT);
r = vioblk_request(vib, xfer, VIRTIO_BLK_T_FLUSH);
r = vioblk_request(vib, xfer, VIRTIO_BLK_T_DISCARD);