[IA64] VTI: fix Oops: time tick before it's due

[IA64] VTI: fix Oops: time tick before it's due

1. Guest may set itm several times in one execution of timer handler of
   guest.  VMM need to handle this situation.
2. VMM don't need to stop guest timer when switching out and rest guest
   timer when switching in, this may make room for some corner case, I don't
   figure out this kind of corner cases now :-), I just removed this logic.
3. When VMM emulate writing itv, VMM can't simply stop timer, when guest
   is masked.
4. All operations such as read/write itv, itc, itm don't need to disable
   interrupt, due to there is no conflict access.

After all these modifications, VTIdomain don't complain "Oops: time tick
before it's due", I don't do the full test:-).

Signed-off-by: Anthony Xu <anthony.xu@intel.com>