Release v0.4.2

Revert "vm: let libvirt choose the MAC address for us"

Turns out the dhcp_release thing was the much better solution – see
commit d0c20c8d9541 ("vm rm: try releasing the VMs DHCP lease") for
details. This actually brings more unexpected problems, and the old
version should work better overall (especially if dhcp_release is not
installed).