pygrub can't boot ubuntu

[Mark Davies <mark%ecs.vuw.ac.nz@localhost>]

Hi,
  I have previously taken a working VirtualBox Ubuntu image, and used
"VBoxManage clonehd" to get a raw diskimage that I've successfully been
able to run under Xen4.6, using pygrub to boot.

I've just tried the same with a Ubuntu 17.4 image but trying to start it
gives the following in a bootloader.log file:

Traceback (most recent call last):
  File "/usr/pkg/bin/pygrub", line 929, in <module>
    raise RuntimeError, "Unable to find partition containing kernel"
RuntimeError: Unable to find partition containing kernel


Any suggestions on whats actually failing here?

cheers
mark