Remove Checkpoint if Backup operation fails

Discussion: https://github.com/abbbi/virtnbdbackup/discussions/146

In some cases backup operation might fail even if a checkpoint has been created.
In this case, virtnbdbackup could attempt to remove the checkpoint.

If it makes sense to remove the checkpoint depend at which state the backup has failed:

startBackup opertion fails: its safe to remove the checkpoint because no data will be transferred