@songI can replicate your issue with a GTX670, works fine with v6.9 and get code 43 after updating to v6.10, I found that this works for me, please try it, edit the XML and in the features section, below </hyperv> add this:
<kvm>
<hidden state='on'/>
</kvm>
Note that if you have "fast startup" enabled in Windows, and it's enabled by default, first boot after the change will still have the problem, you need to re-start the VM (not shutdown and turn on again).
P.S. besides the code 43 issue I also had to dump the VBIOS to use the GTX670 with v6.10, but that's a different issue since there's no display output without it.