Nova 32 bits on older Pentium 4 computer.

[Paul Dufresne]

My current guess is that you don't use virtual 8086 mode but rather,
emulate it in protected mode (as user code) mostly to call (euh run,
much as an assembler interpreter) Vesa BIOS int 0x10 to set video mode
and ask it where it has put the framebuffer.

avl.diff applied.

Much better, from there it should be easy to change video mode depth
to an available one:
[init -> vesa_drv] Found: VESA BIOS version 3.0
[init -> vesa_drv] OEM: Brookdale-G Graphics Chip Accelerated VGA BIOS
[init -> vesa_drv] graphics mode 1024x768 at ...64... not supported
[init -> vesa_drv] Supported mode list
[init -> vesa_drv]     0x109 132x25 at ...167...
[init -> vesa_drv]     0x10a 132x43 at ...167...
[init -> vesa_drv]     0x10b 132x50 at ...167...
[init -> vesa_drv]     0x10c 132x60 at ...167...
[init -> vesa_drv]     0x101 640x480 at ...168...
[init -> vesa_drv]     0x103 800x600 at ...168...
[init -> vesa_drv]     0x105 1024x768 at ...168...
[init -> vesa_drv]     0x111 640x480 at ...64...
[init -> vesa_drv] Searching in default vesa modes
[init -> vesa_drv] Could not set vesa mode 1024x768 at ...64...
[init -> nitpicker] C++ runtime: Genode::Parent::Service_denied
[init -> nitpicker] void* abort(): abort called

Thanks, that is quite fun to see a problem like this being fixed by
exchanging emails!
I guess you don't want the full log. But if you want it, just ask for it.