efi: add helper API for detecting confidential virtualization
This helper is a simplified version of detect_confidential_virtualization()
that merely returns a boolean status flag reflecting whether we are believed
to be running inside a confidential VM.
This flag can be used for turning off features that are inappropriate to
use from a CVM, but must not be used for releasing sensitive data. The
latter must only be done in response to an attestation for the environment.
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>