The clock timer XML is being updated in the following ways (based on
further off-list discussion that was missed during the initial
implementation):

1) 'wallclock' is changed to 'track', and the possible values are 'boot'
 (corresponds to old 'host'), 'guest', and 'wall'.

2) 'mode' has an additional value 'smpsafe'

3) when tickpolicy='catchup', there can be an optional sub-element of
   timer called 'catchup':

   <catchup threshold=123 slew=120 limit=10000/>

Those three values are all longs, always optional, and if they are present,
they are positive. Internally, 0 indicates "unspecified".

* docs/schemas/domain.rng: updated RNG definition to account for changes

* src/conf/domain_conf.h: change the C struct and enums to match changes.

* src/conf/domain_conf.c: timer parse and format functions changed to
                          handle the new selections and new element.

* src/libvirt_private.syms: *TimerWallclock* changes to *TimerTrack*

* src/qemu/qemu_conf.c: again, account for Wallclock --> Track change.