libxl: Fix initialization of nictype in libxl_device_nic

libxl: Fix initialization of nictype in libxl_device_nic

As pointed out by the Xen folks [1], HVM nics should always be set
to type LIBXL_NIC_TYPE_VIF_IOEMU unless the user explicity requests
LIBXL_NIC_TYPE_VIF via model='netfront'.  The current logic in
libxlMakeNic() only sets the nictype to LIBXL_NIC_TYPE_VIF_IOEMU if
a model is specified that is not 'netfront', which breaks PXE booting
configurations where no model is specified (i.e. use the hypervisor
default).

  Reported-by: Stefan Bader <stefan.bader@canonical.com>
[1] https://www.redhat.com/archives/libvir-list/2013-December/msg01156.html