virbr0 as created by libvirt is a private network, it's not a real bridge.
If you want your guests to be reachable, you need to create a real bridge on your virtual hosts and use that as the --virt-bridge in Cobbler.
Instructions are here:
https://fedorahosted.org/cobbler/wiki/VirtNetworkingSetupForUseWithKoan
Making a snippet that automatically makes a bridge for the first interface seems like a nice thing to do (or finding some way to do this with interface objects).
--Michael