If the ultimate goal is to have a Windows 8 Virtual Machine you do not need to have an exiting OS installed to upgrade from using the Upgrade Media. You can clean build using the Windows 8 Upgrade by following information presented in "Activate Windows 8 Pro Upgrade" in Windows 8 Upgrade: clean install possible?.
FWIW I've clean-built a VM of every major release of Windows Desktop OSes using Upgrade Media from Windows 95 onwards. Also as a general rule I never perform in place upgrades on production systems and always clean build, even from upgrade media, and then use Ghost to image to other systems as/when necessary.