libxl: Document device_add_domain_config

libxl: Document device_add_domain_config

Commit 03e1a56d81c16eece735e4d0ef74bfb10eaaba07 replaced DEVICE_ADD()
calls by device_add_domain_config() calls but also removed the comment
of DEVICE_ADD(). Copy the useful part of that comment to
device_add_domain_config().

Also, rename the parameter `type` to `dev`, because that parameter isn't
used as a type but as the device we want to add/update to d_config.

Also, constify `dev` because it isn't modified.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>