Ejemplo de como usar si tenemos un eth0 y un eth1

Creamos un script /etc/xen/scripts/network-multi-bridge

#!/bin/sh
dir=$(dirname "$0")
"$dir/network-bridge" "$@" vifnum=0 bridge=xenbr0 netdev=eth0  
"$dir/network-bridge" "$@" vifnum=1 bridge=xenbr1 netdev=eth1

Dentro de /etc/xen/xend-config.sxp agregamos

(network-script network-multi-bridge)

En la configuracion de nuestra maquina virtual

name = "virtserv"
memory = 2048
vcpus = 1
bootloader = "/usr/bin/pygrub"
on_poweroff = "destroy"
on_reboot = "restart"
on_crash = "restart"
disk = [ "phy:/dev/mapper/VolGroup00-root,hda1,w ]
vif = [ "mac=00:17:4e:37:f5:47,bridge=xenbr0", "mac=00:17:4e:37:f5:48,bridge=xenbr1" ]