Hmm, now that I am more familiar with using libvirt, I think if I were to operate a large rack server, I'd probably roll my own Arch Linux VM host setup and manually setup vms via libvirt. Although anything with snapshots and rollbacks that supports libvirt would do, really. I can see the appeal of something like Proxmox, but now that I know more I'd rather do it myself and avoid overhead.

Just fantasizing about having a server rack again someday...