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: Vincent Palatin <vpalatin@chromium.org>
Message-Id: <20170320101549.150076-1-vpalatin@chromium.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>