Comment 5 for bug 331128

Jamie-

Try adding "-net user" to the end of the kvm command.

So:
 $ /usr/bin/kvm -S -M pc -m 384 -smp 1 -name sec-hardy-i386 -uuid c28d7c53-0da0-75c0-b360-df536d434f51 -monitor pty -pidfile /var/run/libvirt/qemu//sec-hardy-i386.pid -boot c -drive file=/srv/vms/kvm/sec-hardy-i386/root.qcow2,if=ide,index=0,boot=on -net nic,macaddr=00:16:36:70:61:a2,vlan=0,model=virtio -net tap,fd=18,script=,vlan=0,ifname=vnet1 -serial none -parallel none -usb -vnc 127.0.0.1:2 -net user

This is working like a champ for me, with a Jaunty host, and both Hardy and Jaunty guests:
 $ kvm -hda hardy-server.img -cdrom ../iso/ubuntu-8.04.1-server-amd64.iso -net nic,model=virtio -net user

:-Dustin