A bridge should take on the mac address of the hardware device its sharing, and although I cannot see your ifcfg- scripts it looks like you have that correct. But could you post your ifcfg-br0 and the others? One difference I can see is that you use the virtio model nics in your guests and I don't. Also, it looks as though you have a pair of onboard nics, since the mac addresses are so alike. They aren't setup in any sort of gang-mode in the bios? To be honest, I've never looked at the mac addresses of the bridges or vnets to see if they change, but I doesn't make sense that they would...