Merge pull request #18567 (VirtualBox 5.1.6)

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.