Enable virtualisation.writableStore by default

This works around:

  machine: must succeed: nix-store -qR /run/current-system | grep nixos-
  machine# error: changing ownership of path ‘/nix/store’: Invalid argument

Probably Nix shouldn't be anal about the ownership of the store unless
it's trying to build/write to the store.

http://hydra.nixos.org/build/45093872/nixlog/17/raw
(cherry picked from commit 57a0f140643cde409022e297ed05e05f8d34d778)