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>