incomplete emulation of fstenv under TCG

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/661696

Steps to reproduce:

1) Install Windows (tried XP and 7) in qemu (tried qemu without kvm and
qemu-kvm).

2) Get OllyDbg ( http://ollydbg.de/odbg200.zip ).

3) Use some Metasploit-encoded file, example included.

It is not a virus!

File was generated with Metasploit, command (if i remember it right):
`msfpayload windows/exec cmd=notepad R | msfencode -e x86/shikata_ga_nai
-t exe -o cmd_exec_notepad.shikata_ga_nai.exe`.

4) Launch the file under Windows in qemu, make sure it opens a notepad.

5) Open file under OllyDbg, run (F9) it there. It opens a notpad. Close
OllyDbg.

6) Open file under OllyDbg, trace over (Ctrl+F12) it there. It fails with
`Access violation when writing to [some address]`.
Command: 316A 13, XOR DWORD PTR DS:[EDX+13],EBP

Under native Windows, the trace over command works fine.

Under VMware the trace works fine.
Under VirtualBox it also fails (checked in the spring).

$ qemu-kvm --version
QEMU PC emulator version 0.12.5 (qemu-kvm-0.12.5), Copyright (c) 2003-2008
Fabrice Bellard