pci: call object_unparent() before free_qdev()

Start VM with 8 multiple-function block devs, hot-removing
those block devs by 'device_del ...' would cause qemu abort.

| (qemu) device_del virti0-0-0
| (qemu) **
|ERROR:qom/object.c:389:object_delete: assertion failed: (obj->ref == 0)

It's a regression introduced by commit 57c9fafe

The whole PCI slot should be removed once. Currently only one func
is cleaned in pci_unplug_device(), if you try to remove a single
func by monitor cmd.

free_qdev() are called for all functions in slot,
but unparent_delete() is only called for one
function.

Signed-off-by: XXXX
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/acpi_piix4.c

@@ -299,6 +299,7 @@ static void acpi_piix_eject_slot(PIIX4PMState *s, unsigned slots)
             if (pc->no_hotplug) {
                 slot_free = false;
             } else {
+                object_unparent(OBJECT(dev));
                 qdev_free(qdev);
             }
         }

hw/pci.c

@@ -1527,7 +1527,6 @@ static int pci_unplug_device(DeviceState *qdev)
         qerror_report(QERR_DEVICE_NO_HOTPLUG, object_get_typename(OBJECT(dev)));
         return -1;
     }
-    object_unparent(OBJECT(dev));
     return dev->bus->hotplug(dev->bus->hotplug_qdev, dev,
                              PCI_HOTPLUG_DISABLED);
 }