hax: fix breakage in locking

use qemu_mutex_lock_iothread consistently in qemu_hax_cpu_thread_fn() as
done in other _thread_fn functions, instead of grabbing directly the
BQL. This way we ensure that iothread_locked is properly set.

On v2.9.0-rc0, QEMU was dying in an assertion in the mutex code when
running with '--enable-hax' either on OSX or Windows. This bug was triggered
since the code modification for multithreading added new usages of
qemu_mutex_iothread_locked.
This fixes the breakage on both platforms, I can now run again a full
Chromium OS image with HAX kernel acceleration.
Signed-off-by: NVincent Palatin <vpalatin@chromium.org>
Message-Id: <20170320101549.150076-1-vpalatin@chromium.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

cpus.c

@@ -1344,8 +1344,9 @@ static void *qemu_hax_cpu_thread_fn(void *arg)
 {
     CPUState *cpu = arg;
     int r;
+
+    qemu_mutex_lock_iothread();
     qemu_thread_get_self(cpu->thread);
-    qemu_mutex_lock(&qemu_global_mutex);
 
     cpu->thread_id = qemu_get_thread_id();
     cpu->created = true;