Goal

• extend libvirt to allow the use of qcow2 images with blocks stored in an external file via the 'data-file' feature qcow2 provides{}
• ensure incremental backups work in said scenario

Acceptance criteria

• verify that VMs can be started using such setup
• verify that the 'data-file' can point to local files and local block devices
• verify that incremental backups work in such scenario
• verify that there are no regressions from adding this feature