snapshotctl: Check for cow path when allocated from /data

If the COW device is allocated only from /data, then
the COW device name will end with -cow-img. Hence, check
that path as well.

Bug: 335552315
Test: snapshotctl apply-update
Change-Id: Id3c5cf8afd77994da117de41bb98a226b350f8e4
Signed-off-by: Akilesh Kailash <akailash@google.com>