lttng-ctl: Add session shm_path to extended session attributes

lttng-ctl: Add session shm_path to extended session attributes

Observed issue
==============

Once a session is created, any lttng clients have no way of checking
what the current shared memory path setting for a session is.

Solution
========

Expose the configure path via an attribute in the extended structure
of lttng sessions.

Known drawbacks
===============

None.

Change-Id: Ie257bcedfc913e09fc6a60e6b9387d2e8d4cb828
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>