Use nested hashes instead of string keys

This avoids needing to generate a new string value for each key on
setting of the value and retrieving, and the extra hashes are not a huge
consumer of space.

Of note, the keys are now not strings at all.  Top level will be the
class names, and the child keys will be just the raw IDs.  The
@ems_xml_nodes value is not used anywhere besides the lookup that was
changed in this commit, so the change of this structure should not have
an adverse affect... hopefully

app/models/miq_request_workflow.rb

@@ -842,8 +842,12 @@ def find_classes_under_ci(item, klass)
 
   def load_ems_node(item, log_header)
     @ems_xml_nodes ||= {}
-    klass_name = item.kind_of?(MiqHashStruct) ? item.evm_object_class : item.class.base_class.name
-    node = @ems_xml_nodes["#{klass_name}_#{item.id}"]
+    klass_name = if item.kind_of?(MiqHashStruct)
+                   Object.const_get(item.evm_object_class)
+                 else
+                   item.class.base_class
+                 end
+    node = @ems_xml_nodes[klass_name][item.id]
     $log.error("#{log_header} Resource <#{klass_name}_#{item.id} - #{item.name}> not found in cached resource tree.") if node.nil?
     node
   end
@@ -957,7 +961,8 @@ def ems_metadata_tree_add_hosts_under_clusters!(result)
 
   def convert_to_xml(xml, result)
     result.each do |obj, children|
-      @ems_xml_nodes["#{obj.class.base_class}_#{obj.id}"] = node = xml.add_element(obj.class.base_class.name, :object => ci_to_hash_struct(obj))
+      @ems_xml_nodes[obj.class.base_class] ||= {}
+      @ems_xml_nodes[obj.class.base_class][obj.id] = node = xml.add_element(obj.class.base_class.name, :object => ci_to_hash_struct(obj))
       convert_to_xml(node, children)
     end
   end