Isolate NixOS QEMU VMs from each other and from the host by using a private /nix/store image that contains only the VM's dependencies (like the installer has) rather than a virtio mount of the host's entire /nix/store.
Update: virtualisation.useNixStoreImage exists now! But it builds
the store image at runtime, rather that at build-time, drastically
increasing VM start-up time.