spapr: Use error_reportf_err()

Not caught by Coccinelle, because we report the error only
conditionally here.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <1450452927-8346-14-git-send-email-armbru@redhat.com>

hw/ppc/spapr.c

@@ -122,10 +122,11 @@ static XICSState *xics_system_init(MachineState *machine,
             icp = try_create_xics(TYPE_KVM_XICS, nr_servers, nr_irqs, &err);
         }
         if (machine_kernel_irqchip_required(machine) && !icp) {
-            error_report("kernel_irqchip requested but unavailable: %s",
-                         error_get_pretty(err));
+            error_reportf_err(err,
+                              "kernel_irqchip requested but unavailable: ");
+        } else {
+            error_free(err);
         }
-        error_free(err);
     }
 
     if (!icp) {