vr_alloc_ring
static vr_result_t vr_alloc_ring(vr_t *vrp, vr_ring_t *r, size_t n);
if (vr_alloc_ring(vrp, &vrp->rxring, vrp->rx.ndesc) != VR_SUCCESS)
if (vr_alloc_ring(vrp, &vrp->txring, vrp->tx.ndesc) != VR_SUCCESS) {