Data mover fails if a stale volumesnapshot exists in application namespace

We can use a unique label on volumesnapshots for a specific VSB to avoid this issue.

To reproduce, take a DM backup of a namespace where an existing volumesnapshot resource exists. Data Mover will fail on the stale volumesnapshot trying to create a VSB for it.

We want to confirm VSBs only get created for volumesnapshots that are created for the current backup run.