Hi Robert,
I've modified disk images for oVirt virtual machines before. I mostly run servers so they all use preallocated as opposed to thin provisioned disks. I'm not sure if that matters, but it means my VM disk images are raw files (as opposed to qcow). Therefore, I used something like the following guide to get into the disk image. These directions don't use kpartx or libguestfs, but I guess those would work too. I think it goes without saying, your VM should be off before you modify its disk without its knowledge.
For me, the biggest problem I had was the partition in the disk image was really an LVM PV. And that LVM group had the same volume group name as one on the server I was doing this modification. I had two volume groups with the same name, which made things a little tricky. But I'll leave that to you to figure out :)
Scott