qapi, target/i386/sev: Add cpu0-id to query-sev-capabilities

Add a new field 'cpu0-id' to the response of query-sev-capabilities QMP
command.  The value of the field is the base64-encoded unique ID of CPU0
(socket 0), which can be used to retrieve the signed CEK of the CPU from
AMD's Key Distribution Service (KDS).

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220228093014.882288-1-dovmurik@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>