target-ppc: Update error handling in ppc_cpu_realize()

Commit fe828a4d4b added a new fatal error
message while QOM realize'ification was in flight.

Convert it to return an Error instead of exit()ing.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

target-ppc/translate_init.c

@@ -10043,9 +10043,9 @@ static void ppc_cpu_realizefn(DeviceState *dev, Error **errp)
 
 #if !defined(CONFIG_USER_ONLY)
     if (smp_threads > max_smt) {
-        fprintf(stderr, "Cannot support more than %d threads on PPC with %s\n",
+        error_setg(errp, "Cannot support more than %d threads on PPC with %s",
                    max_smt, kvm_enabled() ? "KVM" : "TCG");
-        exit(1);
+        return;
     }
 #endif