For QEMU 7.0.0-rc1, nbd-server-add fails with qcow2 image with iothread in migration context
Description of problem:
Upon adding the drive for NBD (via QMP), there is an error message
````kvm: ../block.c:3657: bdrv_open_child: Assertion `qemu_in_main_thread()' failed.````
and then the process aborts.
Steps to reproduce:
1. Create image: `qemu-img create -f qcow2 /root/target-disk.qcow2 4G`
2. Start QEMU as mentioned above.
3. Issue `nbd-server-start` QMP command (I used type unix).
4. Issue `nbd-server-add` command for the single disk.