libxl: add "merge" function to generic device type support

libxl: add "merge" function to generic device type support

Instead of using a macro generating the code to merge xenstore and
json configuration data, use the generic device type support for
this purpose.

This requires to add some accessor functions to the framework and
a structure for disks (as disks are added separately they didn't need
such a structure up to now).

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>