Ubuntu + bridge +qemu + tap + virtio dropping packets
OK, thanks to the bridge guys, I found the answer. My main problem was that I didn't realize that the "TX queue" means the bridge is transmitting TO the guest (i.e. the Windows virtio driver.) Basically the Windows virtio driver I was using was crashing. I was pointed at these:
https://bugs.launchpad.net/ubuntu/+source/qemu-kvm/+bug/1325560
https://bugzilla.kernel.org/show_bug.cgi?id=42829
I was careful to go to the virtio web site and download the latest drivers, uninstall the ones on my VMs and re-install the latest, and it seems to work.