vm/windows: Reintroduce setting up /nix/store.

Now we're doing this at the point where we're saving the VM state.
Unfortunately it's not quite right, because the controller VM is shut
down _before_ we're saving the state, so the share gets disconnected
despite autodisconnect being deactivated during setup.

We can get around this issue by finally introducing the xchg share,
which is the last item to be implemented before we can merge to master.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>