Remove function replacement with mock patch

In the hyperv unit tests, an rpc method is
manually replaced with a MagicMock using setattr.
This prevents it from being cleaned up by mock.patch.stopall.
This patch replaces it with a short-lived patch call
in a with statement.

Change-Id: I7b10a8115c474977e3acd16dc49d6f3ae67b0c3b
Partial-Bug: #1316401
Merge "Remove function replacement with mock patch"