pci: call object_unparent() before free_qdev()

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: Anthony Liguori <aliguori@us.ibm.com>