Rename _post_live_migration instance_ref arg

This was causing issues with the wrap_instance_fault decorator, as it
expects the argument to be called 'instance'. Also it is being passed an
instance object now so instance_ref can be misleading.

This patch also moves all the tests for the said method to use instance
objects instead of a dict, as this is what is happening in reality.

Finally we secure the wrap_instance_fault decorator with a recently
introduced expects_func_args decorator so that we make sure this issue
does not go unnoticed in the future.

Closes-bug:#1296593
Change-Id: I140bfec2a52bf659a725a7dbe78ba5c527ed26de
Merge "Rename _post_live_migration instance_ref arg"