Bug#960390: x86_64: No serial port output

[Punit Agrawal]

Package: grub2

Grub does not output to serial port when running in a VM launched using
Qemu when run as part of the Debian Installer. This prevents
installation of Debian in a VM when running without a GUI. In this
specific instance, I was trying to install Debian in a VM over ssh.

To reproduce the problem, run

    $ qemu-system-x86_64 -cdrom debian-10.3.0-amd64-netinst.iso -nographic

The BIOS / Firmware outputs to the serial port but no further progress
is seen on the serial console. Dropping the `-nographic` shows that
Debian Installer is launched in a graphical window waiting for input.

Once installed without `-nographic`, the following changes to
/etc/default/grub and running `sudo update-grub` enabled serial output.

    GRUB_TERMINAL=console
    GRUB_SERIAL_COMMAND="serial --speed=11520 --unit=0 --word=8 --parity=no --stop=1

The behaviour was seen on both Buster and Bullseye based Debian
Installer images on x86_64. It was also consistent for both legacy and
UEFI firmware.

Let me know if you need additional information.

Thanks.