* Simultaneous backups. Disabled by default, the number of threads (simultaneous backups) is configurable by tweaking the `threads` option in the config file.
* Cleaner output. Dropped progress bar, which were kind of broken anyway when using compression. Can be re-implemented later, but the progress bars will need to be parallel for the multithreading.
* Use a callback registrer for libvirt events. Allows to add callbacks for different domains but for the same type of events.