Add a config_writer API based on libxml2

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add session configuration XSD

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add a save API to lttng-ctl

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add session configuration save feature to session daemon

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add a save command based on the save API to the lttng client

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