Send a RARP packet after migration.

Currently, after a migration qemu sends a broadcast packet to update
switches' MAC->port mappings.

Unfortunately, it picks a random (constant) ethertype and crosses its
fingers that no one else is using it.

This patch causes it to send a RARP packet instead.  RARP was chosen for
2 reasons.  One, it is always harmless, and will continue to be so even
as new ethertypes are allocated.  Two, it is what VMware ESX sends, so
people who write filtering rules for switches already know about it.

I also changed the code to send SELF_ANNOUNCE_ROUNDS packets, instead of
SELF_ANNOUNCE_ROUNDS + 1, and added a simple backoff scheme.

Signed-off-by: Nolan Leake <nolan <at> sigbus.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
