kubevirt-dm: DeleteItemAction - cleanup of qcow, vmb etc..

[weshayutin]

Description:
We need a DeleteItemAction plugin to handle cleaning up relevant data in the BSL and in cluster. In particular, we need to remove the per-backup manifest and any references to this backup in the per-VM manifests. If we are removing the last backup reference to a checkpoint, then we need to delete this checkpoint's dir and qcow2 files, and if we are removing the last checkpoint for a VM, then we need to delete this VM's manifest and dir.

We should also check for any VMBs for this backup (via label selector) and delete any of these that are still in the cluster.

Subtasks:

DOD: