nixos/qemu-vm: introduce `virtualisation.mountHostNixStore` option•••Now that `useBootLoader` produces a full system image, moving disk
images can be slow because they have a full Nix store in them.
It does not make sense to keep the 9p mountpoint to shadow the
/nix/store of the VM.
We disable it if we have `useBootLoader` and introduce an option for
easy overrides.
origin/qemu-vm/mount-host-nix-store