Moving Xen img to virtualbox failure

I wanted to transfer a xen image with Ubuntu to run under VirtualBox. Because I can control VirtualBox using scripts and the command line, and VMWare which I usually use doesn't think that feature is interesting to us Mac users.

I downloaded then xen test.img file from the server to my Mac.

VboxManage convertdd test.img test.vdi

Create a new virtual machine, use test.vdi as disk.

My mac crashes and reboots!

I try to boot the virtualbox virtual machine up again when the Mac has rebooted. It starts to boot, but says no disk found.

Boot up with a rescue disc installed. It doesn't find a partition table. fdisk -l shows a /dev/sda but also says it doesn't have a valid partition table.

I made one attempt to look for bugs on the Virtualbox site, but didn't find a report. Maybe it is not with VirtualBox? For reference the exact version is VirtualBox Os-X 3.04 r50677

Next strategy is to determine exactly what Ubuntu version the xen vm is running, using lsb_release -r, install the same version as a new virtualbox virtual machine, and then transfer the interesting files from the img file. That will be good enough I think, and probably faster than analyze this problem in depth.