A list of paths whose closure should be made available to the VM.
When 9p is used, the closure is registered in the Nix database in the VM. All other paths in the host Nix store appear in the guest Nix store as well, but are considered garbage (because they are not registered in the Nix database of the guest).
When virtualisation.useNixStoreImage is set, the closure is copied to the Nix store image.