Changeset 201 for trunk

a system for building guest images on demand (partially implemented)

The plan, roughly, is

1. we download an upstream install CD iso, mount it, copy out the contents;
2. when a user wants a VM, the script makes a copy, applies a small static patch (mostly a preseed file for the installer), adds a few vm-dependent variables to the preseed, maybe makes some other changes here, then makes an iso from that;
3. we boot a vm from the new iso, with the empty volume as /dev/hda, and let the installer run.

This checkin implements 2 and 3, when run in an appropriate testing
environment, but I haven't adapted it to the real environment yet.
Step 2 turns out to be surprisingly fast; much less than a second.
Step 3, unfortunately, is looking like 20 minutes, maybe 10 if we
work at tightening it. We'll want to mitigate that somehow.

Code in the 'partial' directory is meant to be moved elsewhere
as it gets integrated.