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

nova/api/openstack/compute/servers.py

@@ -59,6 +59,8 @@ CONF.import_opt('reclaim_instance_interval', 'nova.compute.manager')
 
 LOG = logging.getLogger(__name__)
 
+XML_WARNING = False
+
 
 def make_fault(elem):
     fault = xmlutil.SubTemplateElement(elem, 'fault', selector='fault')
@@ -74,6 +76,12 @@ def make_server(elem, detailed=False):
     elem.set('name')
     elem.set('id')
 
+    global XML_WARNING
+    if not XML_WARNING:
+        LOG.warning(_('XML support has been deprecated and will be removed '
+                      'in the Juno release.'))
+        XML_WARNING = True
+
     if detailed:
         elem.set('userId', 'user_id')
         elem.set('tenantId', 'tenant_id')