vmbuilder: Two fixes to image.xml

1. Create an initial nic, which will give the headnode access to the
   internet.
2. Fix the path to the disk image, so that libvirt uses the copy in
   /var/lib/libvirt/images, rather than the one in the local directory.
vmbuilder: broke some settings out into a config.mak.

In particular, the owner and group of the installed disk image.
Merge branch 'vmbuilder' of github.com:CCI-MOC/haas into vmbuilder