Given that the installer for QEMU Guest Agent is bundled into the Virtio drivers ISO, would it make sense to have the option works as just true/false and reuse the same Virtio sources instead of having to maintain it separately?
I was already able to slightly simplify things with the existing function by just pointing it at a file:// URI so it can load from an existing source instead of having to download it, but being able to just point it at the ISO would make it even easier to keep it updated & aligned.