apt-get install bridge-utils

auto eth0
iface eth0 inet manual

auto xenbr0
iface xenbr0 inet static
    address your.ip.address
    bridge_fd 0
    bridge_stp off
    bridge_ports eth0
    dns-nameserver 8.8.8.8
    gateway your.gateway.address
    netmask your.netmask

modprobe dummy && echo dummy >>/etc/modules

auto dummy0
iface dummy0 inet manual
    pre-up ifconfig $IFACE up
    post-down ifconfig $IFACE down

auto xenbr0
iface xenbr0 inet static
    address ip.address.on.loopback
    bridge_ports dummy0
    bridge_fd 0
    bridge_stp off
    bridge_maxwait 0
    netmask netmask.on.loopback