Although it was written in the context of Xen, you might also want to have a look at the netloop nloopbacks parameter as described in http://www.novell.com/communities/node/4094/xen-network-bridges-explained-with-troubleshooting-notes. On a Xen cluster with 3 physical interfaces per node I had to increase that parameter to keep interfaces from going numb.
I don't know how this translates to the libvirt/kvm world.
Devin