Add session configuration load capability to libconfig

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add load command to the lttng client

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Fix: Fail configuration if libxml2 is not found

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