[Libguestfs] windows xp see 'unknow partition'

Hi, I use guestfs.part_disk to partition one disk with 'mbr' and formated it to ntfs, then attach this disk to one windows xp virtual machine. But this windows xp can't recognise that partition, it says 'uknown partition' Why it is so, and how to solve it? May someone help me with this problem? Thanks.