libsnapshot: MapCowImage returns the COW device path

When a device is not dm-mapped, the whole path of the newly generated
COW device is required.
This was causing some of the libsnapshot_test cases to fail.
Fix by making MapCowImage() optionally return the COW device path.

Fixes: 142800685
Test: libsnapshot_test
Change-Id: I9570b4254d47b2d8fc942053590a320dcb20f7d5
Signed-off-by: Alessio Balsini <balsini@google.com>