s390x/css: provide a dev_path for css devices

We need to implement the get_dev_path method for the css bus, or
else we might end up with two different devices having the same
qdev_path.

This was noticed when adding two scsi_hd controllers: The SCSIBus
code will produce a non-unique dev_path for vmstate usage if the
parent bus does not provide the get_dev_path method.

We simply use the device's bus id, as this is unique and we won't
have any deeper hierarchy from a channel subsystem perspective
anyway.

Note that we need to disable this for older machine versions,
as this changes the migration format.

Reported-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Tested-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>