Don't clone MAC addresses when cloning a libvirt machine
While cloning a libvirt machine, Boxes copies the literal VM XML config and duplicates it into a newly created machine. This way the machines are expected to be identical.
Users won't ever want to have two Boxes with the same mac address.
A common use-case for Boxes is for someone to create and configure a single box and clone it into multiple instances.
Fixes #262 (closed)