qxl: locking fix

One spice worker call lacks the unlock/relock calls,
which may lead to deadlocks, add them.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/hw/qxl.c b/hw/qxl.c
index dcea65d..fe4212b 100644
--- a/hw/qxl.c
+++ b/hw/qxl.c
@@ -866,7 +866,9 @@
     dprint(d, 1, "%s\n", __FUNCTION__);
 
     d->mode = QXL_MODE_UNDEFINED;
+    qemu_mutex_unlock_iothread();
     d->ssd.worker->destroy_primary_surface(d->ssd.worker, 0);
+    qemu_mutex_lock_iothread();
 }
 
 static void qxl_set_mode(PCIQXLDevice *d, int modenr, int loadvm)