libvirt users - Aug 2010 - How to force the use of virtio as the guest network driver

I'm running KVM on a standard Debian Lenny host, and I seem to be 
hitting the problem identified here: 
http://serverfault.com/questions/123566/is-anyone-using-kvm-in-production

My symptom is that using rsync over SSH to transfer files into the guest 
gets slower and slower, and then grinds to a halt, and the machine 
becomes unreachable.

It seems that I should be using the virtio driver, but I can't seem to 
make it load.  The KVM guest has the virtio module, (a la find 
/lib/modules -name \*virtio\*) but when I try to force it by setting 
<model type="virtio"/> in the XML configuration, it seems to
make no
difference.

I see that as per http://libvirt.org/formatdomain.html , "qemu -net 
nic,model=? /dev/null" returns the following: "ne2k_isa i82551 i82557b
i82559er ne2k_pci pcnet rtl8139", but I have no qemu-kvm binary.  Not 
sure if that's a telling symptom or not.

Host kernel is 2.6.26-2-openvz-amd64, KVM version is 72+dfsg-5~lenny5, 
libvirt version is 0.4.6-10, all of which look good enough to use virtio 
according to  http://wiki.libvirt.org/page/Virtio .

Thanks,

Tony Lewis
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