Change errors_out_migration decorator to work with RPC

This decorator in nova/compute/manager.py was written to work with
positional arguments, but at run time the decorated methods are
called via RPC with keyword arguments.  This change fixes the
decorator and the associated test cases.

Closes-Bug: #1300380

Change-Id: I9f87151a71e5217e7eff97cdc5cad2c85b620fa5
Merge "Change errors_out_migration decorator to work with RPC"