Fixes in babeltrace core to support non-CTF traces

Fixes in babeltrace core to support non-CTF traces

Since we will have an effort soon to cleanup the Babeltrace plugin
mechanism, this patch adds just the bits required in Babeltrace to not
crash if a non-ctf is opened. We also add the support of net:// URIs.
That way, an input plugin can just implement the open_trace callback
and process its own input trace without requiring a CTF iterator.

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