tests: qapi-schema tests for allow-preconfig

use new allow-preconfig parameter in tests and make sure that
the QAPISchema can parse allow-preconfig correctly

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1526058959-41425-1-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>