Found a way to get easy NAT / host only virtual networks in oVirt 3.4. I didn't see this documented, so posting it here:

1. Create a fake ethernet card

- ip link set name vmnet0dummy dev dummy0

Now you can create a new network (bridge) named vmnet0, and attach it to vmnet0dummy.

The issue is that as posted on http://www.ovirt.org/Features/Nicless_Network#Benefit_to_oVirt:

I am told that implementing this in Engine is quite a pain, as network external interfaces are currently used as keys. A network without nics is currently unsearchable.

You'll need to figure out your own dhcp / NAT using libvirt or something running on your server, but a *lot* easier and less hacky than http://lists.ovirt.org/pipermail/users/2012-April/001751.html :-)

-- Alex