libxl: Is the nic param to libxl_network_device_add an (in)_out parameter?

[Euan Harris <euan.harris@citrix.com>]

Hi,

If I call libxl_device_nic_add and pass in a mostly-default
libxl_device_nic structure, the function fills in the unspecified default
config fields with data for the NIC which it has just created:

	libxl_device_nic nic;
   	libxl_device_nic_init(&nic);
	/* 
	   nic.devid == -1 
	   nic.mac == 00:00:00:00:00:00 
           nic.model == null
	   etc.
	 */

	libxl_device_nic_add(ctx, domid, &nic, NULL);
	/* 
	   nic.devid == 3 
	   nic.mac == 00:16:3e:1b:7b:12
           nic.model == rtl8139
	   etc.
	 */

Is this behaviour an intentional part of the API which I can rely on,
or just an artefact of the current implementation?  In other words, is
nic meant to be an (in)_out parameter?  If it's not, what is the correct
way to find out the device ID which was allocated for the new device,
for example?

Thanks,
Euan