Thanks, Hanna and Kevin!
I didn't implement bdrv_parse_filename() because I didn't want to invent more legacy syntax. But maybe that legacy syntax does have a use and I just didn't realize it :).
It's easy for io_uring, we can just take the filename and not worry about other parameters (they can largely be set via existing QEMU command-line syntax already).
I'll give it a try.
Stefan