Maybe a OS limitation?
I found this (its not Win8 but...): _http://en.wikipedia.org/wiki/Hyper-V#Graphics_issues_on_the_host
In Windows Server 2008, Microsoft officially supported Hyper-V only with the default VGA drivers, which do not support Windows Aero, higher resolutions, rotation, or multi-monitor display.
I saw some other stuff on google about the max resoution being 1600x1200, might try that. Nothing definitive though.