Add an inventory_collection for the EMS

There are some attributes that need to be updated on the EMS record by
the parser.  For graph refresh this is currently done by directly doing
an ems.update_attributes! in the parser which breaks our "parser
shouldn't connect to the DB" rule.

This adds an inventory collection which allows the parser to set
properties to be saved on the ext_management_system record.

app/models/manageiq/providers/inventory/persister/builder/shared.rb

@@ -37,6 +37,16 @@ def vendor
       'unknown'
     end
 
+    def ext_management_system
+      add_properties(
+        :manager_ref       => %i(guid),
+        :custom_save_block => lambda do |ems, inventory_collection|
+          ems_attrs = inventory_collection.data.first&.attributes
+          ems.update_attributes!(ems_attrs) if ems_attrs
+        end
+      )
+    end
+
     def vms
       vm_template_shared
     end