Add user data for private port

A private port belongs to a private component, so it's okay to set
custom user data within a port. The user must make sure to release the
private user data whatever happens. The easiest solution is to make
a port's private data owned by the component's private data.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>