[libvirt] Using -netdev with qemu/kvm rather than old style

[Pete Ashdown]

I have 0.8.3 installed and my net devices are still being defined via -net
rather than -netdev.  I have found that using the latter allows my ethernet to
properly offload and gives a 35% performance gain.  Looking through the
libvirt code I find in qemu_conf.c:

       /* Possible combinations
        *
        *  1. Old way:   -net nic,model=e1000,vlan=1 -net tap,vlan=1
        *  2. Semi-new:  -device e1000,vlan=1        -net tap,vlan=1
        *  3. Best way:  -netdev type=tap,id=netdev1 -device e1000,id=netdev1 
        *
        * NB, no support for -netdev without use of -device
        */

How do I convince virsh/libvirt to use #3, the "Best way"?