mmm, i have remade some test with 2.6.38 kernel and qemu 0.14.1 , and i can 100% reproduce the bug
netbios copy (40GB files) from a win2003 to win2008R2 (virtio 1.2.0).
after 20-30GB, nic on win2008 does not respond. (but all is fine inside the vm, i have 2 virtual nics, i can connect with the other nic.²
i'll try qemu 0.15 this week.