netdev: Adopt four-step alloc/construct/destruct/dealloc lifecycle.

netdev: Adopt four-step alloc/construct/destruct/dealloc lifecycle.

This is the same lifecycle used in the ofproto provider interface.
Compared to the previous netdev provider interface, it has the
advantage that the netdev top layer can control when any given
netdev becomes visible to the outside world.

Signed-off-by: Ben Pfaff <blp@nicira.com>