viridian: stop directly calling viridian_time_ref_count_freeze/thaw()...

viridian: stop directly calling viridian_time_ref_count_freeze/thaw()...

...from arch_domain_shutdown/pause/unpause().

A subsequent patch will introduce an implementaion of synthetic timers
which will also need freeze/thaw hooks, so make the exported hooks more
generic and call through to (re-named and static) time_ref_count_freeze/thaw
functions.

NOTE: This patch also introduces a new time_ref_count() helper to return
      the current counter value. This is currently only used by the MSR
      read handler but the synthetic timer code will also need to use it.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>