packets: Use RARPs for learning packets.

packets: Use RARPs for learning packets.

Traditionally Open vSwitch had used 802.2 SNAP packets to update
upstream switch learning tables when necessary.  This approach had
advantages in that debugging information could be embedded in the
packet helping hapless admins figure out what's going on.  However,
since both qemu and VMware use RARP for this purpose, it seems
appropriate to fall in line with the defacto standard.

Requested-by: Ben Basler <bbasler@nicira.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>