vring_avail
size = sizeof (struct vring_avail) + sizeof (uint16_t) +
vq->vq_avail = (struct vring_avail *)base;
phys += sizeof (struct vring_avail) + sizeof (uint16_t) +
struct vring_avail *vq_avail; /* the "avail" ring */
size += sizeof(struct vring_avail) + (num * sizeof(uint16_t)) +