network: fix IP address of backend network interface

Get it from settings provided by dom0, do not calculate itself. This
makes a difference for DispVMs.

network/vif-route-qubes

@@ -54,7 +54,7 @@ if [ "${ip}" ] ; then
 		${cmdprefix} ip route ${ipcmd} ${addr} dev ${vif} metric $metric
 	done
 	${cmdprefix} iptables -t raw $iptables_cmd -i ${vif} \! -s ${ip} -j DROP
-	back_ip=${ip%.*}.1
+	back_ip=`xenstore-read qubes-netvm-gateway`
 	${cmdprefix} ip addr ${ipcmd} ${back_ip}/32 dev ${vif}
 fi