Funny I was thinking about a similar script line. Then I thought, this is silly I must have overlooked the obvious. Let's ask the list :-) The machine is dual bootable (Xen/Kvm). It serves as a backup for two other machines running Xen (centos5). That's basically the only reason I'm still on C5. I use drbd to mirror disks. The best approach for me is to take a new machine with C6 and migrate on there.
Funny thing I went through a similar thing a ways back - but for me was vmware esx 3.5 to centos 5.5 (or maybe 5.4 ... whichever was fully supported and not a tech preview) kvm servers in our development environment here.... and now those systems being migrated to new c6 hosts as part of a hardware refresh cycle....
The libvirt/virsh stuff is fairly interesting - a lot is scriptable... but for 'home grown' CLI stuff rather than using something like Archipel or oVirt to manage it you really need to make good use of awk, sed, etc etc to manipulate data as you want.