libxl: Fix devid init in libxlMakeNicList

libxl: Fix devid init in libxlMakeNicList

This basically reverts commit ba64b97134a6129a48684f22f31be92c3b6eef96
"libxl: Allow libxl to set NIC devid". However assigning devid's
before calling libxlMakeNic does not work as that is calling
libxl_device_nic_init which sets it back to -1.
Right now auto-assignment only works in the hotplug case. But even if
that would be fixed at some point (if that is possible at all), this
would add a weird dependency between Xen and libvirt versions.
The change here should accept any auto-assignment that makes it into
libxl_device_nic_init. My understanding is that a caller always is
allowed to make the devid choice itself. And assuming libxlMakeNicList
is only used on domain creation, a sequential numbering should be ok.

Signed-off-by: Stefan Bader <stefan.bader@canonical.com>