Small fix to lttng list session

This patch makes lttng_list_sessions return 0 (not an error) when there
is in fact zero registered sessions. Also adding a MSG() to the user to
inform that no sessions exist and the error code is 0.

Reported-by: Tan Dung Le Tran <tan.dung.le.tran@ericsson.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>