tracetool: show trace-events filename/lineno in fmt string errors

The compiler encounters trace event format strings in generated code.
Format strings are error-prone and therefore clear compiler errors are
important.

Use the #line directive to show the trace-events filename and line
number in format string errors:
https://gcc.gnu.org/onlinedocs/gcc-10.2.0/cpp/Line-Control.html

For example, if the cpu_in trace event's %u is changed to %p the
following error is reported:

  trace-events:29:18: error: format ‘%p’ expects argument of type ‘void *’, but argument 7 has type ‘unsigned int’ [-Werror=format=]

Line 29 in trace-events is where cpu_in is defined. This works for any
trace-events file in the QEMU source tree and the correct path is
displayed.

Unfortunately there does not seem to be a way to set the column, so "18"
is not the right character on that line.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200827142915.108730-5-stefanha@redhat.com>

scripts/tracetool/backend/log.py

@@ -37,12 +37,16 @@ def generate_h(event, group):
     out('    if (%(cond)s && qemu_loglevel_mask(LOG_TRACE)) {',
         '        struct timeval _now;',
         '        gettimeofday(&_now, NULL);',
+        '#line %(event_lineno)d "%(event_filename)s"',
         '        qemu_log("%%d@%%zu.%%06zu:%(name)s " %(fmt)s "\\n",',
         '                 qemu_get_thread_id(),',
         '                 (size_t)_now.tv_sec, (size_t)_now.tv_usec',
         '                 %(argnames)s);',
+        '#line %(out_next_lineno)d "%(out_filename)s"',
         '    }',
         cond=cond,
+        event_lineno=event.lineno,
+        event_filename=event.filename,
         name=event.name,
         fmt=event.fmt.rstrip("\n"),
         argnames=argnames)