nixos/nix-daemon: Require .mount for /nix/store.

Also related to NixOS/nixops#350, because while switching to the new
configuration, depending on /nix/store also propagates to the mount
points for /nix/.ro-store and /nix/.rw-store and we don't get an error
while trying to unmount them (because nix-daemon needs to be stopped for
unmounting these paths).

While Nix does have the option to set a different store path, I've found
only hardcoded references in nix-daemon.nix, so I'm using a hardcoded
reference here as well, because after all customizing the store path
will probably only make sense on non-NixOS systems.

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