I'm using KVM and libvirt on my host (Debian lenny) + 2 bridges per guest (one for mgmt, one for public traffic).
That setup isn't stable at all, sometimes I can do pings to a management ip, sometimes not.
I don't know if my bridging paramateres are correct, could you check ? or if there is anything wrong ...

Please also note that interface on guest doesn't flap and that I got not logs on my host.
Of course forwarding is enabled.

iface eth3 inet manual

auto bond0
iface bond0 inet manual
slaves eth1 eth2
pre-up ip link set bond0 up
down ip link set bond0 down

auto br0
iface br0 inet static
address 10.160.0.7
netmask 255.255.255.128
bridge_ports eth3
bridge_fd 9
bridge_hello 2
bridge_maxage 12
bridge_stp off

auto br0:1
iface br0:1 inet static
address 10.160.0.9
netmask 255.255.255.128

auto br0:2
iface br0:2 inet static
address 10.160.0.10
netmask 255.255.255.128

auto br1
iface br1 inet static
address 217.4.40.242
netmask 255.255.255.240
gateway 217.4.40.241
pre-up /etc/network/firewall start
bridge_ports bond0
bridge_fd 9
bridge_hello 2
bridge_maxage 12
bridge_stp off

auto br1:1
iface br1:1 inet static
address 217.4.40.252
netmask 255.255.255.240

auto br1:2
iface br1:2 inet static
address 217.4.40.253
netmask 255.255.255.240