Stub class object variables.

Create stub class objects at the vvp level and generate the code
to invoke that stub. Implement the routines needed to implement
a test for null object references.

sv_vpi_user.h

@@ -48,6 +48,7 @@ EXTERN_C_START
 #define vpiIntVar           612
 #define vpiByteVar          614
 #define vpiLogicVar         vpiReg
+#define vpiClassVar         615
 #define vpiStringVar        616
 #define vpiBitVar           620
 #define vpiArrayVar         vpiRegArray