This changes `-chardev stdio` to `-chardev file` and uses a temporary
file to communicate with QEMU.
This fixes an issue where `-chardev stdio` hangs if `STDIN` is not a
TTY. I could not figure out how to make it work without a TTY, and it
does not print any meaningful diagnostics. Problem is, in CI and other
automated runners, we do not necessarily have a TTY as STDIN.
This just switches to a temporary file, which seems to work under all
circumstances.