i had the same problem with centos 6.3 on ibm x3650 server. windows xp with virtio net just reset with any network load and on host /var/log/messages says kernel: kvm: 2889: cpu0 unhandled wrmsr: 0x198 data 0 found the solution on proxmox forum. just disable jumbo frames on centos host interface ifcfg and ethernet switch. hope this help someone.