I had this this exact same problem on my ASRock H97M Pro 4. I couldn't get any thing to pass through until I added an old GPU in my primary PCI-E slot. My CPU has no onboard video.
Did you end up leaving the old GPU in? I didn't try removing it and starting the VM, but I have a feeling it won't work without the old one in place for unraid to use.
I did end up leaving it in. If I took it out, even with the GPU I wanted to pass through in the second Pci-E slot, it wouldn't work