trace: skip qemu_set_log_filename if no "-D" option was passed

When the "simple" backend is not active but the "log" backend is,
both "-trace file=" and "-D" will result in a call to
qemu_set_log_filename.  Unfortunately, QEMU was also calling
qemu_set_log_filename if "-D" was not passed, so the "-trace
file=" option had no effect and the tracepoints went back to
stderr.

Fortunately we can just skip qemu_set_log_filename in that case,
because the log backend will initialize itself just fine as soon
as qemu_set_log is called, also in qemu_process_early_options.

Cc: stefanha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210209145759.141231-3-pbonzini@redhat.com>

softmmu/vl.c

@@ -2364,7 +2364,9 @@ static void qemu_process_early_options(void)
 #endif
 
     /* Open the logfile at this point and set the log mask if necessary.  */
-    qemu_set_log_filename(log_file, &error_fatal);
+    if (log_file) {
+        qemu_set_log_filename(log_file, &error_fatal);
+    }
     if (log_mask) {
         int mask;
         mask = qemu_str_to_log_mask(log_mask);