docs/system/i386: Add measurement calculation details to amd-memory-encryption

Add a section explaining how the Guest Owner should calculate the
expected guest launch measurement for SEV and SEV-ES.

Also update the name and links to the SEV API Spec document.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20220217110059.2320497-1-dovmurik@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>