gnt-instance info: fix nic display

In branch-2.1 the conversion of NICs to Tuples has been unified to the
_NICListToTuple function, which is used by both hook generation and
instance info generation. Unfortunately the ip and mac fields got
swapped from what instance info expected, so the mac is shown as ip, and
vice versa. Fixing this.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>