Fix secure delete for thick LVM snapshots

Fix secure delete for thick LVM snapshots

This change modifies the behaviour of the secure delete for thick
LVM snapshots to wipe the underlying COW of the snapshot LV
instead of the snapshot LV itself.

This change is necessary because the snapshot LV does not contain
exactly the same number of writable blocks as the original LV.  The
COW includes header information per COW block that identifies the
device as a COW device as well as the source and destination blocks
for the changed item.  The amount of metadata contained in the COW is
variable based on I/O performed on the snapshot.

This does not change the behavior of secure deletes on thin LVs
or secure deletes on the thick LV snapshot origin.

Closes-Bug: #1191812
Change-Id: I20e02b6c20d5ac539b5b5469e665fc986180f2e9