I had the exact same issue with OS X 10.9 a year ago. I'd even bought a compatible GPU so I could set up VGA passthrough—this wasn't merely for testing, I wanted a 10.9 system for general use, and KVM would have been a way to get it on a modern CPU.
So I have no advice, but if you get it working please do get in touch. I obviously gave up but I'd love to return to it.
