qxl: lock current_async update in qxl_soft_reset

This should fix a defect report from Coverity.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

hw/display/qxl.c

@@ -1156,7 +1156,9 @@ static void qxl_soft_reset(PCIQXLDevice *d)
     trace_qxl_soft_reset(d->id);
     qxl_check_state(d);
     qxl_clear_guest_bug(d);
+    qemu_mutex_lock(&d->async_lock);
     d->current_async = QXL_UNDEFINED_IO;
+    qemu_mutex_unlock(&d->async_lock);
 
     if (d->id == 0) {
         qxl_enter_vga_mode(d);