vm/windows: Collect exit code from xchg.

This is the last item that was missing to get a fully working
runInWindowsVM function. Apart from checking exit codes, we also now
have preVM/postVM hooks which we can use to write arbitrary constructs
around this architecture, without the need to worry about specific
details.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>