meson: document why we don't generate trace events for tests/ and qga/

meson: document why we don't generate trace events for tests/ and qga/

Making trace generation work for tests/ and qga/ would involve some
Meson hackery to ensure we generate the trace-events files before
trace-tool uses them. Since we don't actually support tracing there
anyway, we bypass that problem.

Let's add corresponding comments.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20220126161130.3240892-7-vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Pasto fixed, commit message punctuation tidied up]
Signed-off-by: Markus Armbruster <armbru@redhat.com>