netfront: configure MTU before IP addresses

IP routes read interface MTU on IP address configuration so in case
of DHCP, MTU has to be propagated before other parameters.

This patch fixes the last big MTU-related problem we see on EC2.
Having this patch applied OSv TCP performance on EC2 is pretty much
equal to Linux instances and limited by EC2 networking infrastructure.

Buffer size    Osv          Ubuntu 12.04 HVM
TCP TX
32             5129.27      5128.11
64             5129.18      5128.26
128            5129.33      5133.59
256            5130.63      5128.64
512            5129.4       5133.04
1024           5129.7       5131.87
2048           5128.27      5132.73
4096           5129.57      5133.04
8192           5129.18      5132.16
16384          5129.02      5131.77
32768          5129.36      5132.55
65536          5129.29      5132.05

TCP RX
32             298.26       292.31
64             571.4        540.72
128            1069.91      1014.22
256            1953.42      1925.36
512            3282.79      3261.96
1024           4963.05      5002.54
2048           5127.83      5121.33
4096           5124.61      5127.28
8192           5129.35      5136.58
16384          5128.05      5124.87
32768          5130.63      5135.68
65536          5132.22      5137.49

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>