KVM Ethernet bridge flaky connection and delay

KVM Ethernet bridge flaky connection and delay

libvirt 0.9.10 qemu-kvm CentOS 6, STP enabled bridge. One guest has very
unreliable connection. OS is Win Vista, connection shows ip4 Internet
connected but often 100% packet loss pinging gateway. I run tcpdump on
host when pinging gateway from guest and I dont see the icmp requests
until seconds after the ping timeout. Guest uses br0
bridge name bridge id STP enabled interfaces
br0 8000.90b11c106fec yes em1
vnet0
vnet1
virbr0 8000.525400b7455a yes virbr0-nic