Fix hot remove assigned device regression

Fix hot remove assigned device regression

When hot remove an assigned device, segmentation fault was triggered
by qdev_free(&pci_dev->qdev) in pci_unregister_device().
pci_register_device() doesn't initialize or set pci_dev->qdev. For an
assigned device, qdev variable isn't touched at all. So segmentation
fault happens when to free a non-initialized qdev.

This patch adds a parameter in pci_unregister_device to check if
it's an assigned device. For assgined device, free pci_dev instead of
qdev. No changes for other devices.

Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>