CA-120846: fix get_mac for bonded interfaces

When an interface is bonded by the Linux bonding driver, the driver may change
the MAC address of the interface to that of the bond. The only place where you
can find the "real" MAC address of a bond slave seems to be /proc/net/bonding.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #27 from robhoes/ea1254

CA-120846: fix get_mac for bonded interfaces