libsnapshot: Ensure we can remove bad snapshots when beginning an update.

If somehow we wind up with snapshots with a source suffix, we could wind
up trying to unmap an in-use partition. Detect this case and allow the
snapshot to be deleted without the unmap.

Bug: 183567503
Test: vts_libsnapshot_test
Change-Id: I87dd5bb3a7b9be59dede624924374ccc47b563c2