lttng-live input plugin

This new input format allows Babeltrace to connect a lttng-relayd and
attach to a running live session.

In order to use it, you need to create a live session (refer to LTTng
documentation for that) and then :
babeltrace -i lttng-live net://hostname

Where hostname is the hostname or IP address of the lttng-relayd
receiving the live trace.

When you do this command, it will list the active sessions on the relay.
To attach to a particular session, redo the same command, but add after
the hostname the session ID of the session you want to read.
For example, to attach to the session 42 :
babeltrace -i lttng-live net://hostname/42

If you want to know what is going on in the background, you can add the
-v for a verbose output.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>