If --quiesce is specified, libvirt will try to use guest agent to freeze and unfreeze domain’s mounted file systems. However, if domain has no guest agent, snapshot creation will fail. Currently, this requires --disk-only to be passed as well.
So there could be some check if it was ok or not and if not re-run backup without quiesce option. I agree 
Hi all, I just released my script Contribution: OpenNebula backup script for QCOW2 datastores
Hy Guys,
please check https://wiki.sep.de/wiki/index.php/4_4_3_Tigon:OpenNebula_Backup
and tell me how you like it?
Christian
Hi there Kristian Feldsam,
I’ve been interested in finding a way to do a live backup of a running VM, and then sharing this for other systems to load and run.
Your script has essentially done this for me, and is of great help as I intended to do exactly as Snowman Martin intends; Borg backup.
Martin, have you had success in this method in the half decade since your work? K Feldsam, have you continued working with OpenNebula?
Kind regards,
Filene Taylor B