system/main: transfer replay mutex ownership from main thread to main loop thread

On MacOS, UI event loop has to be ran in the main thread of a process.
Because of that restriction, on this platform, qemu main event loop is
ran on another thread [1].

This breaks record/replay feature, which expects thread running qemu_init
to initialize hold this lock, breaking associated functional tests on
MacOS.

Thus, as a generalization, and similar to how BQL is handled, we release
it after init, and reacquire the lock before entering main event loop,
avoiding a special case if a separate thread is used.

Tested on MacOS with:
$ meson test -C build --setup thorough --print-errorlogs \
func-x86_64-x86_64_replay func-arm-arm_replay func-aarch64-aarch64_replay
$ ./build/qemu-system-x86_64 -nographic -icount shift=auto,rr=record,rrfile=replay.log
$ ./build/qemu-system-x86_64 -nographic -icount shift=auto,rr=replay,rrfile=replay.log

[1] https://gitlab.com/qemu-project/qemu/-/commit/f5ab12caba4f1656479c1feb5248beac1c833243

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/2907
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250410225550.46807-2-pierrick.bouvier@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

system/main.c

@@ -25,6 +25,7 @@
 #include "qemu/osdep.h"
 #include "qemu-main.h"
 #include "qemu/main-loop.h"
+#include "system/replay.h"
 #include "system/system.h"
 
 #ifdef CONFIG_SDL
@@ -44,10 +45,12 @@ static void *qemu_default_main(void *opaque)
 {
     int status;
 
+    replay_mutex_lock();
     bql_lock();
     status = qemu_main_loop();
     qemu_cleanup(status);
     bql_unlock();
+    replay_mutex_unlock();
 
     exit(status);
 }
@@ -67,6 +70,7 @@ int main(int argc, char **argv)
 {
     qemu_init(argc, argv);
     bql_unlock();
+    replay_mutex_unlock();
     if (qemu_main) {
         QemuThread main_loop_thread;
         qemu_thread_create(&main_loop_thread, "qemu_main",