java_gen: add support for properties to specify a custom code template

This will be used for getCanonical()
diff --git a/java_gen/java_model.py b/java_gen/java_model.py
index 2b17afa..c2d43c2 100644
--- a/java_gen/java_model.py
+++ b/java_gen/java_model.py
@@ -983,9 +983,10 @@
 
 class JavaVirtualMember(JavaMember):
     """ Models a virtual property (member) of an openflow class that is not backed by a loxi ir member """
-    def __init__(self, msg, name, java_type, value=None):
+    def __init__(self, msg, name, java_type, value=None, custom_template=None):
         JavaMember.__init__(self, msg, name, java_type, member=None)
         self._value = value
+        self.custom_template = custom_template
 
     @property
     def is_fixed_value(self):