sev/i386: hmp: add 'info sev' command

The command can be used to show the SEV information when memory
encryption is enabled on AMD platform.

Cc: Eric Blake <eblake@redhat.com>
Cc: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Reviewed-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>