Change time policy to eliminate time drift

At the moment we use the following structure:

<clock offset='utc'>
  <timer name='hpet' present='no'/>
</clock>

To elimintate time drift between virtual instances we are changing
structure to:

<clock offset='utc'>
  <timer name='rtc' tickpolicy='catchup' track=‘guest'>
    <catchup threshold='123' slew='120' limit='10000’/>
  </timer>
  <timer name='pit' tickpolicy='delay'/>
  <timer name='hpet' present='no'/>
</clock>

Change-Id: I3e00e7cfb11584d484665a9872ee59318e7209e7
Closes-Bug: 1394546