trace: add --group=all to tracing.txt

trace: add --group=all to tracing.txt

tracetool needs to know the group name ("all", "root", or a specific
subdirectory).  Also remove the stdin redirection because tracetool.py
needs the path to the trace-events file.  Update the documentation.

Fixes: 2098c56a9bc5901e145fa5d4759f075808811685
       ("trace: move setting of group name into Makefiles")
Buglink: https://bugs.launchpad.net/bugs/1844814
Reported-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20191009135154.10970-1-stefanha@redhat.com>