Enabling virtualbox guest addition breaks multi-monitor setup

[rvolosatovs]

Issue description

Setting virtualisation.virtualbox.guest.enable to true breaks dual-monitor setup for me. The output is "duplicated". Every X window is shown on 2 monitors at the same time. Panel is stretched across them.

Steps to reproduce

Set virtualisation.virtualbox.guest.enable to true and attach 2 monitors.

Technical details

 - system: `"x86_64-linux"`
 - host os: `Linux 4.9.78, NixOS, 17.09.git.3e97d0c (Hummingbird)`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 1.11.16`
 - channels(root): `"nixos-17.09.2875.c2b668ee726, nixpkgs-18.03pre126020.931a0b8be80"`
 - channels(rvolosatovs): `""`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs`

Setup in configuration.nix additionally:

  services.xserver.xrandrHeads = [ "DP1" "eDP1" ];
  services.xserver.resolutions = [ { x = 3840; y = 2160; } { x = 1920; y = 1080; } ];

[stale[bot]]

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