Another note to self... Last month, I came across a tool to check your system if it meets 64bit virtualization requirements. Of course, when I needed the tool a few days ago, I couldn't remember where I found it... After going through a lot of mail, I found it again...
But it pointed me in the right direction. The key indeed is to let XP think it's the first hd. But after doing the remapping, grub still sees the original layout, so the above code should be