ctf plugins: prepend trace's hostname, if exists, to trace name

ctf plugins: prepend trace's hostname, if exists, to trace name

This makes the trace names a little more precise.

Note that as of this patch the visitor sets the trace's name just before
adding the first stream class, because after this, the trace is frozen
and we cannot set its name anymore. This is done as lately as possible
to increase our chance of having decoded the metadata's `env` block
(where the hostname is), if any. This can lead to an unnamed trace if
there's no stream classes. But if there's no stream classes, there can't
be any packet or event anyway, so there can't be any notification which
refers to this trace, so no other component should see the fact that
there's no name.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>