Now that overriding fileSystems in qemu-vm.nix works again, it's
important that the VM tests that add additional file systems use the
same override priority. Instead of using the same magic constant
everywhere, they can now use mkVMOverride.
http://hydra.nixos.org/build/6695561
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
