Add /var/emulab/vms to the list of directories we clean.
We also make sure that all dirs in /var/emulab are really dirs and not symlinks. Having /var/emulab/vms be a symlink (left over from a vnode setup) caused vast confusion when trying to use the new images for vnodes. Note: you really, really should not image the vhost in an experiment with vnodes. Prepare in such an environment is destructive and you won't get a clean image anyway (due to LVM state). Swap the Xen image in on a node directly and image that.