viridian: add implementation of synthetic timers

viridian: add implementation of synthetic timers

This patch introduces an implementation of the STIMER0-15_CONFIG/COUNT MSRs
and hence a the first SynIC message source.

The new (and documented) 'stimer' viridian enlightenment group may be
specified to enable this feature.

While in the neighbourhood, this patch adds a missing check for an
attempt to write the time reference count MSR, which should result in an
exception (but not be reported as an unimplemented MSR).

NOTE: It is necessary for correct operation that timer expiration and
      message delivery time-stamping use the same time source as the guest.
      The specification is ambiguous but testing with a Windows 10 1803
      guest has shown that using the partition reference counter as a
      source whilst the guest is using RDTSC and the reference tsc page
      does not work correctly. Therefore the time_now() function is used.
      This implements the algorithm for acquiring partition reference time
      that is documented in the specifiction.

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