vio_ctrl_start
r = vio_ctrl_start(sc, VIRTIO_NET_CTRL_RX, cmd, 1, &slot);
r = vio_ctrl_start(sc, VIRTIO_NET_CTRL_MQ,
r = vio_ctrl_start(sc, VIRTIO_NET_CTRL_GUEST_OFFLOADS,
r = vio_ctrl_start(sc, VIRTIO_NET_CTRL_MAC,
int vio_ctrl_start(struct vio_softc *, uint8_t, uint8_t, int, int *);