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
(cherry picked from commit 72cd430e971589f7872a9d4e8f173c68b4955375)
Merge "Change errors_out_migration decorator to work with RPC" into stable/icehouse