virtualbox-guestadditions: fix xorg incompatibility

A recent X update broke VirtualBox guest additions (vboxvideo driver version
mismatch, desktop won't start). This fixes it.

Here is the error log:

 (II) "glx" will be loaded by default.
 (II) LoadModule: "glx"
 (II) Loading /nix/store/kzvmnjlps51q4piqmwr7zbmxcg2z9vgk-xorg-server-1.13.4/lib/xorg/modules/extensions/libglx.so
 (II) Module glx: vendor="X.Org Foundation"
 	compiled for 1.13.4, module version = 1.0.0
 	ABI class: X.Org Server Extension, version 7.0
 (==) AIGLX enabled
 Loading extension GLX
 (II) LoadModule: "vboxvideo"
 (II) Loading /nix/store/4kbxi00h8xsmfgbws2qqh674lcfp03h6-VirtualBox-GuestAdditions-4.2.14-3.2.46/lib/xorg/modules/drivers/vboxvideo_drv.so
 (II) Module vboxvideo: vendor="Oracle Corporation"
 	compiled for 10.12.0, module version = 1.0.1
 	Module class: X.Org Video Driver
 	ABI class: X.Org Video Driver, version 12.0
 (EE) module ABI major version (12) doesn't match the server's version (13)
 (II) UnloadModule: "vboxvideo"
 (II) Unloading vboxvideo
 (EE) Failed to load module "vboxvideo" (module requirement mismatch, 0)
 (II) LoadModule: "vboxmouse"
 (WW) Warning, couldn't open module vboxmouse
 (II) UnloadModule: "vboxmouse"
 (II) Unloading vboxmouse
 (EE) Failed to load module "vboxmouse" (module does not exist, 0)
 (EE) No drivers available.

Fatal server error:
 no screens found