KVM: selftests: Move Hyper-V VP assist page enablement out of evmcs.h

Hyper-V VP assist page is not eVMCS specific, it is also used for
enlightened nSVM. Move the code to vendor neutral place.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20221101145426.251680-41-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>