viridian: remove duplicate union types

viridian: remove duplicate union types

The 'viridian_vp_assist', 'viridian_hypercall_gpa' and
'viridian_reference_tsc' union types are identical in layout. The layout
is also common throughout the specification [1].

This patch declares a common 'viridian_page_msr' type and converts the rest
of the code to use that type for both the hypercall and VP assist pages.

Also, rename 'viridian_guest_os_id' to 'viridian_guest_os_id_msr' since it
also is a union representing an MSR value.

No functional change.

[1] https://github.com/MicrosoftDocs/Virtualization-Documentation/raw/live/tlfs/Hypervisor%20Top%20Level%20Functional%20Specification%20v5.0C.pdf

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Roger Pau Monne <roger.pau@citrix.com>