Update VirtualBox (and implicitly VirtualBox Guest Additions) to 4.3.6
and Oracle VM VirtualBox Extension Pack to 91406
Conflicts due to minor upgrade in the mean time
Conflicts:
nixos/modules/virtualisation/virtualbox-guest.nix
pkgs/applications/virtualization/virtualbox/default.nix
pkgs/applications/virtualization/virtualbox/guest-additions/default.nix