Add hpet option for time drifting

Since havana, the timezone of windows vm is localtime.
However, there is a time drifting when the windows vm is overloaded.
Disabling hpet option of clock in xml template,
we can sync time without buffering.

From this template,

<clock offset='utc'>
<timer name='pit' tickpolicy='delay'/>
<timer name='rtc' tickpolicy='catchup'/>
</clock>

To this template,

<clock offset='localtime'>
<timer name='rtc' tickpolicy='catchup'/>
<timer name='pit' tickpolicy='delay'/>
<timer name='hpet' present='no'/>
</clock>

Implements : blueprint add-no-hpet-option-into-guest-clock
Change-Id: I6441b9d2de5897dd1863b53bd7e2578881b53c67
Merge "Add hpet option for time drifting"
Merge commit 'refs/changes/97/66597/4' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch