Fix typo in the XML serialization os-services API.

Partially implements blueprint nova-api-samples
fixes bug 1130609

The XML serializer for the update method in the os-services API extensions was
not serializing the "updated_at" field because of a typo in the code.
In this change we added api_samples for the XML output.

Change-Id: I9fff0677e9bad650b19e559b3ed6e9bc0ffe8c3c
(cherry picked from commit 48b41e0b880adf80e3be6d128cd392af57b8477e)