Comment 16 for bug 331128

I believe virtio may be a red herring, this appears to be a problem with any bridge set up using /etc/libvirt/qemu/networks/default.xml

By replacing it with default0.xml default1.xml (where %d is replaced with the appropriate number), changing my vm's virsh XML to use those instead of default, then running 'virsh net-undefine default' and then running net-define + net-start for each of the new defaultX.xml's I was able to start some VM's.. the model type emulated appeared to have no effect.

Note: Some of my attempted fixes caused CPU sucking infinite loops, and others disassociated libvirt from the running kvm, so backup your disk images before trying to get qemu/kvm/libvirt running in jaunty.