Bug#873604: cloud.debian.org: grub shouldn't assume gfxterm

[Mariusz Mazur <mmazur@axeos.com>]

I just tested this:

GRUB_TERMINAL_INPUT="console serial"
GRUB_TERMINAL_OUTPUT="console serial"
GRUB_SERIAL_COMMAND="serial --speed=115200 --unit=0 --word=8
--parity=no --stop=1"

And it worked fine with grub actually showing up with 'virsh console'
and no problems booting under openstack either. This would likely be
the preferred solution.