viridian: define type for the 'virtual VP assist page'

viridian: define type for the 'virtual VP assist page'

The specification [1] defines a type so we should use it, rather than just
OR-ing and AND-ing magic bits.

No functional change.

NOTE: The type defined in the specification does include an anonymous
      sub-struct in the page type but, as we currently use only the first
      element, the struct declaration has been omitted.

[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: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>