nixos/qemu-vm: Allow building a Nix store image instead of using 9p
nixos/qemu-vm: Allow building a Nix store image instead of using 9p

Add the `useNixStoreImage` option, allowing a disk image with the
necessary contents from the Nix store to be built using
make-disk-image.nix. The image will be mounted at `/nix/store` and
acts as a drop-in replacement for the usual 9p mounting of the host's
Nix store.

This removes the performance penalty of 9p, drastically improving
execution speed of applications which do lots of reads from the Nix
store. The caveats are increased disk space usage and image build
time.

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