Remove XML support from some extension v3 API plugins

Remove XML support from the following v3 compute API plugins:

 - extended_server_attributes
 - extended_status
 - extended_volumes
 - extension_info

Part of blueprint remove-v3-xml-api

DocImpact

Change-Id: I4f3785fbdac7257370458a3bdd859d46ef83f415
Merge "Remove XML support from some extension v3 API plugins"