Mark XML as deprecated in the v2 API

Log a warning the first time a server gets encoded as XML.
I didn't see an immediately obvious place to put this warning,
but anyone actually using XML will hit this code path, so it
seems fine to me.

Implements blueprint deprecate-xml-in-v2-api

DocImpact

Change-Id: I4acc15ce5f487738bb34a95c2261a5d05d827d8d
Merge "Mark XML as deprecated in the v2 API"