tests: Add tests for registering instancemethod

Event class cannot be used with instance methods, so we must use partial
holding weak reference to the object. This code is used in StoragePool,
and these tests validate that this works and serve as example code for
Event.

Change-Id: Iba3d4626e3fffab9778364966bc82711c5f29472
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35438
Reviewed-by: Adam Litke <alitke@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>