Fix V3 API server metadata XML serialization

The XML serialisation and deserialization when updating
a single server metadata item or retrieving a single server metadata
item through the V3 API is broken as the JSON format was changed
but the corresponding XML serialisation/deserialisation wasn't.

This changeset fixes this bug and adds testcases for the fixes.

Change-Id: I1bd2a729744387337f86a37dc2ff89606edce424
Closes-Bug: #1223803
Merge "Fix V3 API server metadata XML serialization"