netdev: Add new "struct netdev_rx" for capturing packets from a netdev.

Separating packet capture from "struct netdev" means that there is no
remaining per-"struct netdev" state, which will allow us to get rid of
"struct netdev_dev" (by renaming it "struct netdev").

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>