xengnt_status
int xengnt_status(grant_ref_t);
while (xengnt_status(sc->sc_tx_ring_gntref))
while (xengnt_status(sc->sc_rx_ring_gntref))
KASSERT(xengnt_status(req->txreq_gntref) == 0);
while (xengnt_status(sc->sc_ring_gntref))
KASSERT(xengnt_status(gntref) == 0);
KASSERT(xengnt_status(xbdreq->req_indirect->in_gntref)