Accept domain names for XenServer

As the old script did, the new script should also be able to use a
xenserver if that's specified by a domain name. The IP address will be
detected by looking up xenbr0's address.
Comment out FLAT_NETWORK_BRIDGE

As the change that enables to specify FLAT_NETWORK_BRIDGE hasn't landed
yet, it should be commented out from the official script, and used only
on the internal builds.