viridian: unify time sources

viridian: unify time sources

Currently, the time_ref_count enlightened time source maintains an offset
such that time is frozen when the domain paused, but the reference_tsc
enlightened time source does not. After migrate, the reference_tsc source
may become invalidated (e.g. because of host cpu frequency mismatch) which
will cause Windows to fall back to time_ref_count. Thus, the guest will
observe a jump in time equivalent to the offset.

This patch unifies the two enlightened time sources such that the same
offset applies to both of them. Also, it's not really necessary to have
two different functions to calculating a 10MHz counter value, time_now() and
raw_trc_val(), so this patch removes the latter implementation. The
unification also allows removal of the reference_tsc_valid flag.

Whilst in the area, this patch also takes the opportunity to constify a few
pointers which were missed in earlier patches.

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