This article at ModernAdmin saved me alot of time. I was looking how to add multiple IPs to a Xen DomU in routed mode, this requires routes to be added on the Dom0. One way was to add multiple interfaces to the DomU, the other way is to add the addresses as aliases to an existing interface. You can then just add the addresses in the config file like this:
vif = [ 'ip=10.0.1.1 10.0.1.2 10.0.1.3' ]
vif-route will create all the necessary routes while you need only one virtual network interface.