io_uring / iothread regression 7.1.0
Description of problem:
After upgrading to 7.1.0, some of my libvirt VM's failed to boot. I have narrowed down the issue to the combination of:

- io_uring
- iothread
Steps to reproduce:
1. set up a VM with iothread and io_uring
2. try to boot and watch it "hang"
Additional information:
Here's the relevant command line from the libvirt log:
```
-blockdev '{"driver":"file","filename":"/mnt/data/VMs/Arch-Linux-x86_64-basic.qcow2","aio":"io_uring","node-name":"libvirt-1-storage","auto-read-only":true,"discard":"unmap"}' \
-blockdev '{"node-name":"libvirt-1-format","read-only":false,"driver":"qcow2","file":"libvirt-1-storage","backing":null}' \
-device '{"driver":"virtio-blk-pci","iothread":"iothread1","bus":"pci.4","addr":"0x0","drive":"libvirt-1-format","id":"virtio-disk0","bootindex":1 }' \
```