trap bounce flags

["Jan Beulich" <jbeulich@novell.com>]

With the severe stability issues we are having with SLE10sp1 on x86-64, things
start pointing pretty closely at the int80 direct trap patch we imported from
-unstable. While I just now realized that there's been a fix for these problems
for quite a while (don't know how this slipped my attention), I still have a few
notes:

- even compat_restore_all_guest now asserts interrupts are disabled, despite
  32-bit restore_all_guest not doing so (and the iret path not generally needing
  this)
- int80_direct_trap checks for non-zero TRAPBOUNCE_flags, yet
  {,compat_}create_bounce_frame clear the low byte of these flags (i.e.
  including TBF_exception, which is in this lower byte); it appears to be only a
  lucky coincidence that this still works as the cmp (again!) is suffix-less and
  hence gets sized as a 32-bit compare, accidentally covering TRAPBOUNCE_cs
- from the above, why is it that only the lower byte (if anything) needs clearing?

Jan