Force tap's MAC prefix to "fe"

Setting a high prefix discourages the bridge from adopting the tap's
MAC. Xen is not affected by this since the MAC is forced to
"fe:ff:ff:ff:ff:ff".

This addresses issue #217.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
(cherry picked from commit be3f52f3)
(this is critical enough that we want it on stable-2.6)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>