python3: Fix crashes in snapshot()

This makes sure all PyObject structs are always initialized to NULL,
this will fix issues such as (issue #239).

Also add a snapshot/list/restore testcase to the python3 api test code.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>