Stop QEMU from creating trace- files by disabling trace from default build

Shame but it's way too annoying.

README.adoc

@@ -10891,6 +10891,14 @@ TODO test PyCharm: https://stackoverflow.com/questions/51982735/writing-gem5-con
 
 === Tracing
 
+The tracing described in this section requires rebuilding QEMU with:
+
+....
+./build-qemu --extra-config-args=--enable-trace-backends=simple
+....
+
+Ideally we would like to use this build option by default, but unfortunately starting on some QEMU version this started generating `trace-<pid>` files for every run which is far too annoying when you don't want traces, and we don't know how to turn it off at runtime or make the files be created in some other directory.
+
 QEMU can log several different events.
 
 The most interesting are events which show instructions that QEMU ran, for which we have a helper:

build-qemu

@@ -38,7 +38,9 @@ Extra arguments to pass to configure
             self.sh.run_cmd(
                 [
                     os.path.join(self.env['qemu_source_dir'], 'configure'), LF,
-                    '--enable-trace-backends=simple', LF,
+                    # Ideally we'd want this, but it creates trace-<pid>
+                    # files on cwd which is too annoying.
+                    #'--enable-trace-backends=simple', LF,
                     '--target-list={}'.format(target_list), LF,
                     '--enable-sdl', LF,
                 ] +