linux: Enable Virtualbox guest modules

The mainline Linux kernel modules for VirtualBox guests are enabled by
many other distros. It seems reasonable to assume that they are stable
and functional enough at this point. The modules include vboxguest,
vboxvideo and vboxsf. So enable them for NixOS too.

This provides an overview about what other Linux distros do:

* [Arch](https://archlinux.org/packages/?sort=&q=virtualbox&maintainer=&flagged=)
  * [Enabled mainline](https://gitlab.archlinux.org/archlinux/packaging/packages/linux/-/commit/4ce5aa26d45f2ec30ffb926f3450ae225226dc41) Virtualbox guest kernel modules
  * No guest modules package, only guest utils

* [Alpine](https://pkgs.alpinelinux.org/packages?name=*virtualbox*&branch=edge&repo=&arch=x86_64&origin=&flagged=&maintainer=)
  * [Enabled mainline](https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/main/linux-lts/lts.x86_64.config?__goaway_challenge=cookie&__goaway_id=6d9c6ce73df12ad9d4edd231da8d2b12&__goaway_referer=https%3A%2F%2Fgithub.com%2F#L2750) Virtualbox guest kernel modules
  * no guest modules package, only guest utils

* [openSUSE](https://software.opensuse.org/search?q=virtualbox&baseproject=openSUSE%3AFactory)
  * [Enabled mainline](https://software.opensuse.org/package/kernel-default) Virtualbox guest kernel modules
  * 3rd-party modules are shipped with host modules in one package
    * [Stable](https://software.opensuse.org/package/virtualbox-kmp-default)
    * [LTS](https://software.opensuse.org/package/virtualbox-kmp-longterm)

* [Debian](https://packages.debian.org/search?suite=sid&searchon=names&keywords=virtualbox)
  * [Enabled mainline](https://salsa.debian.org/search?search=vbox&nav_source=navbar&project_id=18670&group_id=2107&search_code=true&repository_ref=debian%2Flatest) Virtualbox guest kernel modules
  * No guest modules package, only guest utils
  * [File search for vboxguest.ko](https://packages.debian.org/search?suite=bookworm&arch=any&mode=exactfilename&searchon=contents&keywords=vboxguest.ko), only shipped by the kernel package

* [Gentoo](https://packages.gentoo.org/packages/search?q=virtualbox)
  * [Enabled mainline](https://github.com/projg2/fedora-kernel-config-for-gentoo/blob/6.12.8-gentoo/kernel-x86_64-fedora.config#L8486) Virtualbox guest kernel modules
  * [Packaged 3rd-party](https://packages.gentoo.org/packages/app-emulation/virtualbox-guest-modules) kernel modules for guests

* [Fedora](https://packages.fedoraproject.org/search?query=virtualbox)
  * [Enabled mainline](https://src.fedoraproject.org/rpms/kernel/c/5d7cf4582847852950c9650e68943241cf68468a

) Virtualbox guest kernel modules
  * No guest modules package, only guest utils

Signed-off-by: Felix Singer <felixsinger@posteo.net>