So I did all the passthrough set up and I launch my VM, change to the DVI monitor (which is now the GPU passthrough yeaaa!).
From the boot menu I select the W10 ISO and start the W10 installation, I choose custom installation and there are no disks available to install W10 on.
I have downloaded the stable virtio ISO with drivers and mounted the ISO on a separate CD-ROM which is visible in the W10 installation, but when I browse to the iostor directory and select “install” nothing happens, Windows installer complains about missing signed drivers.
UPDATE! Wow - you have to point to the AMD64 directory?!? why oh why? Anyway, works now!