Fix default_prefix behavior
Fix default_prefix behavior

Call `dup` on default prefix before amending it via `concat` and
`<<`. Previously when default_prefix was used, it was amended in place,
so it was changing with every VM definition and kept getting longer,
leading to wrong domain names.

vagrant-libvirt/vagrant-libvirt,electrofelix/vagrant-libvirt,infernix/vagrant-libvirt,infernix/vagrant-libvirt,pradels/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,infernix/vagrant-libvirt,pradels/vagrant-libvirt,electrofelix/vagrant-libvirt,electrofelix/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt