datapath: Move vport init to first port create.

datapath: Move vport init to first port create.

vport->init and exit() functions are defined by gre and netdev vport
only and both can be moved to first port create.

Following patch does same, it moves vport init to respective vport
create and gets rid of vport->init() and vport->exit() functions.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>