libxl: Introduce a template for devices with a controller

libxl: Introduce a template for devices with a controller

We have several outstanding patch series which add devices that have
two levels: a controller and individual devices attached to that
controller.

In the interest of consistency, this patch introduces a section that
sketches out a template for interfaces for such devices.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Chun Yan Liu <cyliu@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>