qemu-storage-daemon --nbd-server fails with "too many connections" error

This libnbd interoperability test started failing with qemu-6.0.0-0.3.rc4.fc35.x86_64: https://gitlab.com/nbdkit/libnbd/-/blob/master/interop/interop-qemu-storage-daemon.sh

The qsd command it runs is:

qemu-storage-daemon --blockdev qcow2,file.driver=file,file.filename=qemu-storage-daemon-disk.qcow2,node-name=disk0 --nbd-server addr.type=fd,addr.str=4 --expor nbd,node-name=disk0,id=nbd0,name=,writable=on

The error message printed is:

qemu-storage-daemon: --nbd-server addr.type=fd,addr.str=4: socket_get_fd: too many connections: Invalid argument