i386/sev: Implement ConfidentialGuestSupport functions for SEV

i386/sev: Implement ConfidentialGuestSupport functions for SEV

The ConfidentialGuestSupport object defines a number of virtual
functions that are called during processing of IGVM directives to query
or configure initial guest state. In order to support processing of IGVM
files, these functions need to be implemented by relevant isolation
hardware support code such as SEV.

This commit implements the required functions for SEV-ES and adds
support for processing IGVM files for configuring the guest.

Signed-off-by: Roy Hopkins <roy.hopkins@randomman.co.uk>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/7145835f729e6195f2fbda308aa90e089a96ae6e.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>