spapr_pci.c: add 'unplug already in progress' message for PCI unplug

Hotunplug for all other devices are warning the user when the hotunplug
is already in progress. Do the same for PCI devices in
spapr_pci_unplug_request().

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20210226163301.419727-5-danielhb413@gmail.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr_pci.c

@@ -1743,6 +1743,10 @@ static void spapr_pci_unplug_request(HotplugHandler *plug_handler,
                 }
             }
         }
+    } else {
+        error_setg(errp,
+                   "PCI device unplug already in progress for device %s",
+                   drc->dev->id);
     }
 }