ofp-prop: Add support for putting and parsing nested properties.
It hadn't occurred to me before that any special support was actually
necessary or useful for nested properties, but the functions introduced in
this commit are nice wrappers to deal with the extra 4-byte padding that
ensures that the nested properties begin on 8-byte boundaries just like
the outer properties.
Signed-off-by: Ben Pfaff <blp@ovn.org> Acked-by: Jarno Rajahalme <jarno@ovn.org>