API Fix : handle id to use the public functions

API Fix : handle id to use the public functions

The API functions were designed to be used with struct trace_handle as
parameters, but no public function returns such a struct.
This patch fixes that by allowing the user to pass the context and the
handle_id.

Acked-by: Yannick Brosseau <yannick.brosseau@gmail.com>
Signed-off-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>