virtualbox: Build with PulseAudio by default

If people want to disable support for PulseAudio they can still
explicitly use pulseaudio = false in their nixpkgs config.

But even with enabled PulseAudio support, it's still optional, enabled
at runtime and can be turned off in VirtualBox settings as well.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Issue: #15005