qdev: skip bus check for bus-less devices in qdev_unplug()

Since commit 2f7bd829db "qdev: Fix device_add bus assumptions"
it's possible to device_add bus-less device, but if such device is
unplugged it will dereference NULL parent_bus in qdev_unplug().

Fix it by taking in account that parent_bus might be NULL and
skipping bus check.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-id: 1367587536-14964-1-git-send-email-imammedo@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
