datapath-windows: netdev-windows fixes for vswitchd bringup.

Added the handlers for update_flags and set_etheraddr.
These handlers were needed for vswitchd bringup on windows
platform.

Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>