staging: nvec: Add is_event() and size() functions for nvec_msg

Add functions nvec_msg_is_event() and nvec_msg_size() which
do just what the say: tell whether the message is an event,
and getting the size of the message.

Signed-off-by: Julian Andres Klode <jak@jak-linux.org>
Acked-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>