xen: netfront: explicitly generate arp_notify event after migration.

Use newly introduced netif_notify_peers() method to ensure a gratuitous ARP is
generated after a migration.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Stephen Hemminger <shemminger@linux-foundation.org>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Cc: xen-devel@lists.xensource.com
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>