vProtect is not open source but if you have 10 VMs or less the license is free.  It's an enterprise grade backup solution that can backup oVirt/RHEV VMs (including incremental backups), also handles restores (can mount on the backup machine or restore to a new VM directly to oVirt), snapshot management and a bunch of other things.  I've been demoing it for the past several months to backup my oVirt 4.3 VMs and it has been working great.