[HVM] Enable more than one platform timer (PIT/RTC/HPET)

[HVM] Enable more than one platform timer (PIT/RTC/HPET)
programmed as periodic timer and adds them to abstract layer, which
keeps track of pending_intr_nr to avoid time interrupt lost and
sync'ed timer with TSC.

It also makes some cleanup to the time related code.

Signed-off-by: Xiaowei Yang <xiaowei.yang@intel.com>
Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>