There was one step during the in-place upgrade, when there was a backup of dom0 created in case something went wrong. Where do I find this?
Moderation note: edited for clarity when splitting topics.
The upgrade tool takes a snapshot of the root volume in case the upgrade fails. From what I can see, it deletes the snapshot in the post-upgrade tasks (stage 5). If you’ve gone through all the stages, you won’t be able to find it.