slirp: add a fake NC-SI backend

NC-SI (Network Controller Sideband Interface) enables a BMC to manage
a set of NICs on a system. This model takes the simplest approach and
reverses the NC-SI packets to pretend a NIC is present and exercise
the Linux driver.

The NCSI header file <ncsi-pkt.h> comes from mainline Linux and was
untabified.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>