Prevent re-download virtio-win-0.1.285.iso

[eval-exec]

Hello, thanks for the project, it helps me a lot.

In below screenshot, you can see I execute make build-windows-2022-libvirt, and it downloaded virtio-win-0.1.285.iso, but my system is missing 7z. then I installed 7z, and execute make build-windows-2022-librt again, then it re-download virtio-win-0.1.285.iso again.

I think we can check if we have downloaded the iso file, if it exists, then we don't need to re-download it again.

Thank you.