Use the interface name, not the bridge name when looking up MAC

This is for firstboot and for when xe-reset-networking has been
executed.

Acked-by: Rob Hoes <rob.hoes@citrix.com>
Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>
CA-116420: when creating linux bond, add MAC address after adding bond slaves

This is needed under kernel 3.x, because the kernel now changes the bond MAC to
the MAC of the first slave that is added, which is not always what we want.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>