There is already a section with various SEV commands / types,
so move the SEV guest attestation together.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20211007161716.453984-3-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>