Virsh/virt-manager uses ssh to connect to master, there it expects openbsd netcat(which
has support for unix sockets) to be avalible, to make a tunnel.
Close#1087.
It requires a writable /nix/store to store the build result. Also,
wait until we've reached multi-user.target before doing the build, and
do a sync at the end to ensure all data to $out is properly written.
http://hydra.nixos.org/build/6496716
