ppc/xics: rework the ICS classes inheritance tree

With the previous changes, we can now let the ICS_KVM class inherit
directly from ICS_BASE class and not from the intermediate ICS_SIMPLE.
It makes the class hierarchy much cleaner.

What is left in the top classes is the low level interface to access
the KVM XICS device in ICS_KVM and the XICS emulating handlers in
ICS_SIMPLE.

This should not break migration compatibility.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr.c

@@ -137,7 +137,7 @@ static ICSState *spapr_ics_create(sPAPRMachineState *spapr,
         goto error;
     }
 
-    return ICS_SIMPLE(obj);
+    return ICS_BASE(obj);
 
 error:
     error_propagate(errp, local_err);