Take http://vcpanel.net/ for example, it has an admin back end with a client front end. In short, I don't care about the admin back end. Just a place for users to manage their assigned VM's.
I think DTC could work for you then. The basic idea behind it is:
dom0: Install a small DTC SOAP server to control the VMs.
A spare domU, or some other server: Install the main DTC control panel (which also can handle standard shared web hosting etc.). This client interface allows you to reboot, shut down, kill and see stats on VMs, as well as reinstall VM Operating Systems.
We use it at GigaTux and currently only handle VPS customers. It does the job nicely.