qdev: fix device_del by refactoring reference counting

Commit 8eb0283 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: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
