Add XML deserializer for qos_manage delete_keys API

XML deserializer is missing for delete_keys API resulting XML format
request not being correctly handled by QoS Manage API extension. This
patch adds XML deserializer for the API.

Also add unittest for qos_manage XML serializer and deserilizer.

Change-Id: I7b3ac6822c52f11b08d767aa55b7107bd0333c36
Closes-bug: #1312553
Merge "Add XML deserializer for qos_manage delete_keys API"