xn_get_rx_ref
ref = xn_get_rx_ref(rxq, *cons);
grant_ref_t ref = xn_get_rx_ref(rxq, *cons);
ref = xn_get_rx_ref(rxq, *cons + frags);
ref = rxq->grant_ref[requeue_idx] = xn_get_rx_ref(rxq, i);