Thanks Paul, what a life-saver: I never expected something like that would exist, so I
didn't even look for it.
In the mean-time I went for option three using the 2.5Gbit NIC, because I managed to find
out why my CentOS nodes failed: Adding Cinnamon breaks oVirt for some very strange
reason.
I'd still like to know how to include a driver into an oVirt node image, but with two
of three options working I feel very relaxed and pleased.