stubdom/grub: update init_netfront() call for mini-os

stubdom/grub: update init_netfront() call for mini-os

This patch updates the call of init_netfront() function according to its
recently updated declaration which can also include parameters for gateway
and netmask addresses. While we are here, the patch also removes passing
the ip parameter because (a) it is not used anywhere and (b) it wastes
memory since it would reference a dynamically allocated string.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>