Report bond speed as function of slaved nics speed and bond mode

Currently bond interfaces are always assumed to have 1GB of speed,
reflecting in inaccurate network usage calculations.

This patch add bond speed probe to netinfo.speed(), considering bond
mode and slaved nics speed:

 Failover modes: 1 and 3
  Bond speed = active slaved nic speed.

 Load Balance modes: 0, 2, 4, 5 and 6
  Bond speed = all slaved nics speed summed up.

Change-Id: Ide077846e49ac5e4d759a85ff9d5c6cec653aa18
Bug-Url: https://bugzilla.redhat.com/1007860
Signed-off-by: Amador Pahim <apahim@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19297
Reviewed-by: Dan Kenigsberg <danken@redhat.com>