nbd: Mark fd handlers client type as "external"

So we could distinguish it from internal used fds, thus avoid handling
unwanted events in nested aio polls.
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

nbd.c

@@ -1446,7 +1446,7 @@ static void nbd_set_handlers(NBDClient *client)
 {
     if (client->exp && client->exp->ctx) {
         aio_set_fd_handler(client->exp->ctx, client->sock,
-                           false,
+                           true,
                            client->can_read ? nbd_read : NULL,
                            client->send_coroutine ? nbd_restart_write : NULL,
                            client);
@@ -1457,7 +1457,7 @@ static void nbd_unset_handlers(NBDClient *client)
 {
     if (client->exp && client->exp->ctx) {
         aio_set_fd_handler(client->exp->ctx, client->sock,
-                           false, NULL, NULL, NULL);
+                           true, NULL, NULL, NULL);
     }
 }