[HVM][VMX] Fix for CR8 acceleration on 64bit guest.

[HVM][VMX] Fix for CR8 acceleration on 64bit guest.

For current CR8 acceleration, we do not call update_tpr_threshold() at
every VMEXIT. But at some situations, we cannot inject guest
interrupts in time. And at some critical time, it will bring up a blue
screen to 64bit Windows guest.

Now, we select to call update_tpr_threshold() at very VMEXIT
time. It's safe, and we do not see clear performance downgrade so
far.

Signed-off-by: Xiaohui Xin xiaohui.xin@intel.com