confidential guest support: Update documentation

confidential guest support: Update documentation

Now that we've implemented a generic machine option for configuring various
confidential guest support mechanisms:
  1. Update docs/amd-memory-encryption.txt to reference this rather than
     the earlier SEV specific option
  2. Add a docs/confidential-guest-support.txt to cover the generalities of
     the confidential guest support scheme

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>