netlink-socket: Add support for async notification on Windows.

We keep an outstanding, out of band, I/O request in the driver at all time.
Once an event generated the driver queues the event message, completes the
pending I/O and unblocks the calling thread through setting the event in the
overlapped structure in the NL socket. The thread will read all all event
messages synchronously through the call of nl_sock_recv()

Signed-off-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Samuel Ghinet <sghinet@cloudbasesolutions.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean at cloudbasesolutions.com>
Acked-by: Saurabh Shah <ssaurabh@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>