block/export: call blk_set_dev_ops(blk, NULL, NULL)

Most export types install BlockDeviceOps pointers. It is easy to forget
to remove them because that happens automatically via the "drive" qdev
property in hw/ but not block/export/.

Put blk_set_dev_ops(blk, NULL, NULL) calls in the core export.c code so
the export types don't need to remember.

This fixes the nbd and vhost-user-blk export types.

Fixes: fd6afc5 ("nbd/server: Use drained block ops to quiesce the server")
Fixes: ca858a5 ("vhost-user-blk-server: notify client about disk resize")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20230502211119.720647-1-stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit de79b52)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

block/export/export.c

@@ -192,6 +192,7 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)
     return exp;
 
 fail:
+    blk_set_dev_ops(exp->blk, NULL, NULL);
     blk_unref(blk);
     aio_context_release(ctx);
     if (exp) {
@@ -219,6 +220,7 @@ static void blk_exp_delete_bh(void *opaque)
     assert(exp->refcount == 0);
     QLIST_REMOVE(exp, next);
     exp->drv->delete(exp);
+    blk_set_dev_ops(exp->blk, NULL, NULL);
     blk_unref(exp->blk);
     qapi_event_send_block_export_deleted(exp->id);
     g_free(exp->id);

block/export/vduse-blk.c

@@ -346,7 +346,6 @@ static void vduse_blk_exp_delete(BlockExport *exp)
 
     blk_remove_aio_context_notifier(exp->blk, blk_aio_attached, blk_aio_detach,
                                     vblk_exp);
-    blk_set_dev_ops(exp->blk, NULL, NULL);
     ret = vduse_dev_destroy(vblk_exp->dev);
     if (ret != -EBUSY) {
         unlink(vblk_exp->recon_file);