spapr: disable vmdesc submission for old machines

Since QEMU 2.3, we have a vmdesc section in the migration stream.
This section is not mandatory but when migrating a pseries-2.2
machine from QEMU 2.2, you get a warning at the destination:

qemu-system-ppc64: Expected vmdescription section, but got 0

The warning goes away if we decide to skip vmdesc as well for
older pseries, like it is already done for pc's.

This can only be observed with -cpu POWER7 because POWER8
cannot migrate from QEMU 2.2 to 2.3 (insns_flags2 mismatch).
Signed-off-by: NGreg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr.c

@@ -2452,6 +2452,7 @@ DEFINE_SPAPR_MACHINE(2_3, "2.3", false);
 static void spapr_machine_2_2_instance_options(MachineState *machine)
 {
     spapr_machine_2_3_instance_options(machine);
+    machine->suppress_vmdesc = true;
 }
 
 static void spapr_machine_2_2_class_options(MachineClass *mc)