bootloader v0.11 broken under bochs and qemu/kvm

[Stary2001]

Two issues here:

• enter_unreal_mode should be the very first thing in stage2 main, because the screen write call touches some memory above 64k (movzx ebx, byte ptr ds:[ecx] with ecx=0x13ccc)
  error output in bochs: 00017458470e[CPU0 ] read_virtual_checks(): read beyond limit

• Rust generates some code that uses ss for accesses (mov eax, dword ptr ss:[ebp+edi*8+4], where ss is the default for ebp), and enter_unreal_mode only sets up ds. Unsure how this should be fixed properly - set ss to 32 bit as well?
  error output in bochs: 00695082304e[CPU0 ] stackPrefetch(4): access [0x00013ff8] > SS.limit [0x0000ffff]

As for why this works in qemu in emulation mode: qemu is really inaccurate :(

[phil-opp]

Thanks a lot for reporting these issues! Your proposed fixes sound good.

[Stary2001]

Do you want me to make a PR?

[phil-opp]

That would be great!