Commit 8eb02831 broken device_del by having too overzealous reference counting
checks.  Move the reference count checks to qdev_free(), make sure to remove
the parent link on free, and decrement the reference count on property removal.
Reported-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>