I know this is an old ticket, but it still hasn't been addressed. According to d3vi1 the problem is because Windows requires VGA video (from PCAT BIOS) in addition to GOP/UGA (from [U]EFI). Since VGA video is already implemented in VirtualBox (when using BIOS) and UEFI already supports multiple video systems (GOP and UGA), wouldn't it be easy (for someone with knowledge about them) to simply allow VGA to be accessed while in EFI?