I had a similar problem, I tried to mount the virtio.iso along with the win16.iso and failed because the windows installer did not accept an unsigned virtio.iso. Just install windows. I had to take care of the HW configuration: Hard Disk sata0, 1CD/DVD Drive with the windows.iso (ide0) and 1 CD/DVD with virtio.iso (ide2).
Once windows is istalled follow up the instruction from the video.