xen-netfront: rearrange netfront structure to separate tx and rx

Keep tx and rx elements separate on different cachelines to prevent
bouncing.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Acked-by: Jeff Garzik <jgarzik@pobox.com>
Cc: Stephen Hemminger <shemminger@linux-foundation.org>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>