Cool => I'll then probably reformat on my root server the partition hosting the images of my VMs from EXT4 to XFS => then to make 100% backups from time to time I would just have to shut them down for a minute, do a "cp --reflink" of their image-file on the host/dom0, then I could start them up again and do anytime later the slow download of their "copied" file.

Any recommendations about a utility (working on a layer higher than the filesystem) that can synchronize only the chunks of a specific file that has changed since the last sync and which generates as final result a "normal" file (no behind-the-curtains database - just copy the identical data from the local/old file and the changes/new data from the remote location to get a normal up-to-date local file)?