qdev: Add a get_dev_path() function to BusInfo This function is meant to provide a stable device path for buses which are able to implement it. If a bus has a globally unique addresses scheme, one address level may be sufficient to provide a path. Other buses may need to recursively traverse up the qdev tree. Signed-off-by: Alex Williamson <alex.williamson@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>