This introduces VirtualBox version 5.1.6 along with a few refactored
stuff, notably:
* Kernel modules and user space applications are now separate
derivations.
* If config.pulseaudio doesn't exist in nixpkgs config, the default is
now to build with PulseAudio modules.
* A new updater to keep VirtualBox up to date.
All subtests in nixos/tests/virtualbox.nix succeed on my machine and
VirtualBox was reported to be working by @DamienCassou (although with
unrelated audio problems for another fix/branch) and @calbrecht.
1781e95577