pseries: Restore support for total vcpus not a multiple of threads-per-core for old machine types

As of pseries-2.7 and later, we require the total number of guest vcpus to
be a multiple of the threads-per-core.  pseries-2.6 and earlier machine
types, however, are supposed to allow this for the sake of migration from
old qemu versions which allowed this.

Unfortunately, 8149e29 "pseries: Enforce homogeneous threads-per-core"
broke this by not considering the old machine type case.  This fixes it by
only applying the check when the machine type supports hotpluggable cpus.
By not-entirely-coincidence, that corresponds to the same time when we
started enforcing total threads being a multiple of threads-per-core.

Fixes: 8149e2992f

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>

hw/ppc/spapr.c

@@ -2863,7 +2863,13 @@ static void spapr_core_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
         goto out;
     }
 
-    if (cc->nr_threads != smp_threads) {
+    /*
+     * In general we should have homogeneous threads-per-core, but old
+     * (pre hotplug support) machine types allow the last core to have
+     * reduced threads as a compatibility hack for when we allowed
+     * total vcpus not a multiple of threads-per-core.
+     */
+    if (mc->has_hotpluggable_cpus && (cc->nr_threads != smp_threads)) {
         error_setg(errp, "invalid nr-threads %d, must be %d",
                    cc->nr_threads, smp_threads);
         return;