I had the chance to try the rest of the commands linked by @tuxen today, and now I can successfully run a VM with MxGPU enabled and started! It looks like adding "pci=assign-busses" to this command did it.

/opt/xensource/libexec/xen-cmdline --set-dom0 "pci=realloc pci=assign-busses" 

Thank you all for you assistance!