tests: make lttng_live_server.py print session URLs

tests: make lttng_live_server.py print session URLs

To help with interactive use, make lttng_live_server.py print session
URLs that you can pass directly to babeltrace.

For instance:

    $ PYTHONPATH=/home/smarchi/src/babeltrace/tests/utils/python /opt/python/3.4.10/bin/python3 lttng_live_server.py --trace-path-prefix /home/smarchi/src/babeltrace/tests/data/ctf-traces  split_metadata.json --port 1245
    Listening on port 1245
    net://localhost:1245/host/hostname/split_metadata

Change-Id: Ib4d806804bfdc00132ec7987c62016ad976b50d2
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10900
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>