XO should delete the backup then.
We are currently completely rewriting the backup code, we'll test more once this is released.
These VDI snapshots are attached to VM snapshots which is normal.
XO keeps at least a snapshot per backup for each VM if:
Unfortunately, XO does not currently provide a way to select disks across multiple VMs to migrate them.
It may be possible in the next major release scheduled for September, but not today.
If you are a paying customer, please open a support ticket and we should be able to help you with that.
It's easier for us and it will be handled much more quickly