x86: make Viridian support optional

x86: make Viridian support optional

Add config option HVM_VIRIDIAN that covers viridian code within HVM.
Calls to viridian functions guarded by is_viridian_domain() and related macros.
Having this option may be beneficial by reducing code footprint for systems
that are not using Hyper-V.

Signed-off-by: Sergiy Kibrik <Sergiy_Kibrik@epam.com>
Reviewed-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Acked-by: Jan Beulich <jbeulich@suse.com>