virtualbox: include Guest Additions ISO

Although virtualbox has code to download it at runtime, I'd like to have
it included at build time because:

* I like programs that are fully functional even when offline.
* Runtime downloading doesn't work, it results in error message
  "During certificate downloading: Unknown reason".

This change increases the closure size 5 % (969 -> 1020 MiB).

pkgs/applications/virtualization/virtualbox/default.nix

@@ -4,6 +4,7 @@
 , qttools, qtsvg, qtwayland, pkg-config, which, docbook_xsl, docbook_xml_dtd_43
 , alsa-lib, curl, libvpx, nettools, dbus, substituteAll, gsoap, zlib
 , yasm, glslang
+, linuxPackages
 # If open-watcom-bin is not passed, VirtualBox will fall back to use
 # the shipped alternative sources (assembly).
 , open-watcom-bin
@@ -215,6 +216,7 @@ in stdenv.mkDerivation {
 
     mkdir -p "$out/share/virtualbox"
     cp -rv src/VBox/Main/UnattendedTemplates "$out/share/virtualbox"
+    ln -s "${linuxPackages.virtualboxGuestAdditions.src}" "$out/share/virtualbox/VBoxGuestAdditions.iso"
   '';
 
   preFixup = optionalString (!headless) ''