Fix Xorg-server on vmware inconjunction with fix to

xf86videovmware[1].

Adds "vmware" to list of default options of
services.xerver.videoDrivers.

new default:
 [ "ati" "cirrus" "intel" "vesa" "vmware" ]

old default:
 [ "ati" "cirrus" "intel" "vesa" ]

[1] Pull request for xf86videovmware found at
https://github.com/NixOS/nixpkgs/pull/338.