Since 2012 (commit ba6212d8 "Eliminate cpus-x86_64.conf file") we
have no default config files that would be disabled using
-nodefconfig.  Update documentation and document -nodefconfig as
deprecated.

Cc: Markus Armbruster <armbru@redhat.com>
Acked-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20171004030025.7866-3-ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>