[Libguestfs] guestfs_list_partitions with vdi

[Skippy VonDrake]

I'm using a Windows raw disk image that is "converted" to the VDI
format by custom code that prepends a header to the image.

I use 'vdi' for GUESTFS_ADD_DRIVE_OPTS_FORMAT of
guestfs_add_drive_opts().
After launching the appliance, the call to guestfs_list_partitions()
results in an empty list.
Is there a way to add the drive but have an offset into the image
file as a "starting point"?
So that it could skip the header which apparently is causing the
issue.

If I use the image without the vdi customization (and use 'raw' for
add_drive_opts) then list_partitions is successful.