[IA64] Fix Windows Timer stop issue.

[IA64] Fix Windows Timer stop issue.

When doing HCT testing on Guest Windows, Windows Timer might stop,
if there is no mouse activity.

When implementing vhpi acceleration, I didn't think of below situation.
windows uses "epc" instruction to implement system call like linux,
In a very small code sequence including "epc" instruction, if there is
an external interrupt, Windows will defer handling this external interrupt
by rfi with ipsr.i=0.

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