hw/nvme: unregister the event notifier handler on the main loop

Make sure the notifier handler is unregistered in the main loop prior to
cleaning it up.

Fixes: 2e53b0b ("hw/nvme: Use ioeventfd to handle doorbell updates")
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Jinhao Fan <fanjinhao21s@ict.ac.cn>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>

hw/nvme/ctrl.c

@@ -4311,6 +4311,7 @@ static void nvme_free_sq(NvmeSQueue *sq, NvmeCtrl *n)
     if (sq->ioeventfd_enabled) {
         memory_region_del_eventfd(&n->iomem,
                                   0x1000 + offset, 4, false, 0, &sq->notifier);
+        event_notifier_set_handler(&sq->notifier, NULL);
         event_notifier_cleanup(&sq->notifier);
     }
     g_free(sq->io_req);
@@ -4701,6 +4702,7 @@ static void nvme_free_cq(NvmeCQueue *cq, NvmeCtrl *n)
     if (cq->ioeventfd_enabled) {
         memory_region_del_eventfd(&n->iomem,
                                   0x1000 + offset, 4, false, 0, &cq->notifier);
+        event_notifier_set_handler(&cq->notifier, NULL);
         event_notifier_cleanup(&cq->notifier);
     }
     if (msix_enabled(&n->parent_obj)) {