[Libguestfs] guestfs_list_partitions with vdi

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.