1. Delete VolumeSnapshot directly when DeletionPolicy set to Retain.
2. Change VolumeSnapshotContent's DeletionPolicy to Retain, then delete VolumeSnapshot. After that delete VolumeSnapshotContent and change VSC DeletionPolicy to Delete back, then re-create the VolumeSnapshotContent.
Signed-off-by: Xun Jiang <jxun@vmware.com>