I received the following error message trying to create a Windows 7 virtual machine on Windows 8:
Failed to add device ‘Microsoft Synthetic Ethernet port’

I found people saying to disable your anti-virus which I did and still received the error.
On a whim I renamed the title of the Windows 7 virtual machine from Windows 7 to Windows7 with no space.
For some reason that fixed it and it created the virtual machine.  Weird, but easy fix.