Source: https://forum.proxmox.com/threads/generic-solution-when-install-gets-framebuffer-mode-fails.111577/
cd /usr/share/X11/xorg.conf.d/
touch radeon.conf
vi radeon.conf
file content:
Section "Device"
Identifier "Card0"
Driver "fbdev"
BusID "0b:00.0"
EndSection
xinit -- -dpi 96 >/dev/tty2 2>&1
apt install qemu-guest-agent