ioemu: HVM virtual S3

ioemu: HVM virtual S3
  - add S3 suspend logic in PM1A control register. when guest write
    specific value to this register,
    QEMU will trigger S3 sleep by
    * reset all qemu device
    * set CMOS shutdown status as S3 resume, so that rombios will do
      S3 resume later
    * request Xen to S3-suspend the guest

Signed-off-by: Yu Ke <ke.yu@intel.com>
Signed-off-by: Liping Ke <liping.ke@intel.com?
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>