Don't rely on random interfaces order

[ezbik]

      q_num0 = os.listdir("/sys/class/net/").index(opts.interface) * 2
      q_num1 = os.listdir("/sys/class/net/").index(opts.interface) * 2 + 1

Queues numbers are determined based on os.listdir("/sys/class/net/") which returns array elements in random manner.
Yet it badly covers the situation when interfaces appear\disappear from the OS. We may end up at collisions, when running multiple per-interface Osfooler instances.

I believe we should read /sys/class/net/$if_name/ifindex and use the value for calculating Queue numbers, instead of using array element index numbers.