blockdev driver=file,filename=/dev/block forbidden?

[Michael Tokarev]

Commit 8d17adf34f501ded65a106572740760f0a75577c
"block: remove support for using "file" driver with block/char devices"
explicitly forbids usage of file driver for block devices.

But _why_?

Hasn't we always used file for everything on *nix? And what's the _actual_
difference between file and host_device?

Please note this change has been added to qemu long before libvirt has
been adapted (I guess there's no released libvirt can be used with
qemu 6.0+).

Why?

Thanks,

/mjt