Change backing type of lttng_uuid to std::array

Change backing type of lttng_uuid to std::array

Changing the backing type of lttng_uuid to std::array allows us to
return lttng_uuid from a function. This, in return, makes it possible to
initialize const attributes from the return value of a function
returning a UUID.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie092eab4a848a41ddd9c63f779514f1e4ca2a441