x86/viridian: update to version 5.0a of the specification

x86/viridian: update to version 5.0a of the specification

The Hypervisor Top Level Functional Specification v5.0a has many differences
from previous versions and introduces whole new sections.

This patch:

- Updates the URL at the top of the source.
- Fixes up section references accordingly.
- Modifies the MSR naming convention in the code to match the specification.
- Rename the apic_assist page to the vp_assist page to reflect the change
  in the specification.
  (The APIC assist feature itself is inconsistently named in the
  specification so stick wth the current feature name).
- Updates the handling of CPUID leaf 3.

There is one functional change in this patch: The vp_assist page is
mapped (and completely zeroed) regardless of whether the APIC assist
feature is enabled. This reflects its new wider remit and simplifies the
code slightly.

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