Revert object-assuming changes to _post_live_migration()

Change I140bfec2a52bf659a725a7dbe78ba5c527ed26de converted the
_post_live_migration() call to assume it was passed an object
because the post_live_migration_at_destination() method uses them.
However, it is also called from live_migration() on the source,
which clearly is still using instance dicts.

This patch reverts the changes that assume an instance object,
as well as the tests that were changed as a result. It also
changes the clear_events_for_instance() call to use dict syntax,
since that is called from the live migration path currently.

(cherry picked from commit 887d214186631d3d1f49451fa72feb11251d2176)
Change-Id: I6e915035bc2c1b890e606d17a4195b88c3f26b13
Closes-bug: #1302334