datapath-windows/Netlink: Nested attributes put/parse.

Added APIs for creating and parsing nested netlink attributes.
APIs are on similar lines as userspace netlink code.

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