libxl: nic type defaults to vif in hotplug for hvm guest

libxl: nic type defaults to vif in hotplug for hvm guest

We don't support plugging in emulated nic to a HVM guest.

The "update_json" flag is only set when doing hotplug, so use that as an
indicator in libxl__device_nic_add. The new hotplug flag to _setdefault
function should be false in all other locations.

This then requires saving nic type in JSON file, because we don't want
the receiving end to recalculate the nic type.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>