viridian: use viridian_map/unmap_guest_page() for reference tsc page

viridian: use viridian_map/unmap_guest_page() for reference tsc page

Whilst the reference tsc page does not currently need to be kept mapped
after it is initially set up (or updated after migrate), the code can
be simplified by using the common guest page map/unmap and dump functions.
New functionality added by a subsequent patch will also require the page to
kept mapped for the lifetime of the domain.

NOTE: Because the reference tsc page is per-domain rather than per-vcpu
      this patch also changes viridian_map_guest_page() to take a domain
      pointer rather than a vcpu pointer. The domain pointer cannot be
      const, unlike the vcpu pointer.

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