KVM-Guests can't get past bridge - no internet connection
1.) You're putting eth0 and eth1 into a bonding group and you're bridging them into br0 - you can't do both. Have you tried putting bond0 in instead?
2.) Also - why do you have STP on? Unless you're trying to bridge external segments together in a redundant topology or are you trying to connect a bonded interface to your VM's? Turn it off unless you need it.