Data mover backup failed to delete the created snapshot in some failure case

[Lyndon-Li]

CSI snapshot is created
As one step of Exposing, the created VSC is set to Retain, so we need to delete the VSC explicitly
In some particular cases when Exposing fails, the deletion of VSC doesn't work, VSC will be left

[Lyndon-Li]

This is due to fix of #7068, we added a finalizer to the VSC, but in failure cases, the finalizer is not removed, so the VSC cannot be deleted.
We need to refactor the code so that both #7068 and this issue can be fixed.