x86: Detect shadow-emulate write to stack from inside the write

x86: Detect shadow-emulate write to stack from inside the write
handler, where we can quite conveniently check whether the access
segment is SS. This will only give a false positive for non-stack
writes that override the destination segment to SS, which it is
probably safe to assume will never happen when the destination is a
legitimate page table!

For now, instead of bailing and unshadowing just increment a perfctr
and we'll see if that increases fast under any reasonable workload.

Signed-off-by: Keir Fraser <keir@xensource.com>