s390x/pci: assert zpci always existing

If one pci device is plugged successfully, there must be a zpci device
existing. This means that during hot-unplugging a pci device, its
corresponding zpci device must be found. Therefore we use an assert to
replace current code.
Signed-off-by: NYi Min Zhao <zyimin@linux.vnet.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/s390-pci-bus.c

@@ -718,11 +718,7 @@ static void s390_pcihost_hot_unplug(HotplugHandler *hotplug_dev,
                 break;
             }
         }
-
-        if (!pbdev) {
-            object_unparent(OBJECT(pci_dev));
-            return;
-        }
+        assert(pbdev != NULL);
     } else if (object_dynamic_cast(OBJECT(dev), TYPE_S390_PCI_DEVICE)) {
         pbdev = S390_PCI_DEVICE(dev);
         pci_dev = pbdev->pdev;