`qemu -serial stdio` leaves stdout in non-blocking mode
Description of problem:
When `-serial stdio` is used, qemu exits leaving stdout in non-blocking mode. Although it [attempts](https://gitlab.com/qemu-project/qemu/-/blob/1a2d52c7fcaeaaf4f2fe8d4d5183dccaeab67768/chardev/char-stdio.c#L52) to restore stdin to blocking mode, it misses that stdout also gets O_NONBLOCK by [qemu_chr_open_fd](https://gitlab.com/qemu-project/qemu/-/blob/1a2d52c7fcaeaaf4f2fe8d4d5183dccaeab67768/chardev/char-stdio.c#L116) ([here](https://gitlab.com/qemu-project/qemu/-/blob/1a2d52c7fcaeaaf4f2fe8d4d5183dccaeab67768/chardev/char-fd.c#L215)). It causes the next applications in the script misbehave because they get unexpected EAGAIN on write to stdout.
Steps to reproduce:
Run the following script:

```
#!/usr/bin/env bash

qemu-system-x86_64 -nodefaults -display none -no-reboot -serial stdio &
PID="$!"
sleep 5
kill "$PID"
wait "$PID"
echo "EXITING $?"

sleep 5
seq 1 400000
```

The seq command will be interrupted prematurely:

```
...
5143
5144
5145⏎                                                                                                                                                                                                                wResource temporarily unavailable
write: Resource temporarily unavailable
write: Resource temporarily unavailable
```

When run from fish shell, it will also start misbehaving when running next commands (fish bug report: https://github.com/fish-shell/fish-shell/issues/10600).
Additional information:
Expect a patch from me soon.