vm: implement recommended clock settings

A new set of recommended clock settings emerged as follows:

  <clock ...>
    <timer name="rtc" tickpolicy="catchup"/>
    <timer name="pit" tickpolicy="delay"/>
    <timer name="hpet" present="no"/>
  </clock>

THose settings are recommended by both libvirt and qemu developers,
and they apply for all guest OSes.
Some of them don't affect some OSes, but in those cases they are harmless.

VDSM used to support just the first setting, tickpolicy 'catchup'
for rtc, conditioned by the presence of the 'tdf' option.
Now this setting is always enabled.

This patch updates the XML generation according with
the new recommendations, and drop all the references
to the 'tdf' variable.

API deprecation notice:
the 'tdf' parameter is no longer honored;
however, no supported engines seems to send it anymore.

Change-Id: Icb0752e54a4cb9ff609b8ddfaf5c8fe2ed5b9e72
Bug-Url: https://bugzilla.redhat.com/1053846
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23663
Reviewed-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>