Split seed VM configuring out to a new script.

We can't depend on creating the seed VM fresh in the gate environment
- we don't have permission to create VMs as that would let use create
arbitrary libvirt containers unless very carefully controlled.

Having a one-time operation is also a bit easier to reason about:
 - make the VM
 - later make images for it

At any point in time, the VM exists, rather than being deleted and
recreated.

Change-Id: I91e61fdb51afd33a31eb2d3cad01ba8ac019eb59