This reverts commit df645c22cc.
KVM 1.1.0 breaks several VM builds. The NixOS installer tests hang:
http://hydra.nixos.org/build/2763935
but worse, some builds give random failures indicating corruption:
http://hydra.nixos.org/build/2776221
http://hydra.nixos.org/build/2776180
http://hydra.nixos.org/build/2769347
http://hydra.nixos.org/build/2769337