Fix babeltrace iterator lib: seek at time 0

When seeking by timestamp, we assumed that the timestamp passed as an
argument was a real timestamp from the trace. However LTTV uses
timestamp 0 to refer to the beginning of the trace. This fix ensures
that we read at least the first event after the seek.

Signed-off-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>