fix(vm_image_util): Do not generate a machine UUID.

If QEMU is given a uuid systemd will detect that and in turn use it for
the machine-id. This made the bug causing the machine-id to be always
re-generated on boot harder to notice since it didn't happen on QEMU.
Merge pull request #207 from marineam/fix

fix(vm_image_util): Do not generate a machine UUID.