All XTS configuration uses qemu_private_xts.  Drop other variables as
they have only ever been used to generate the summary (which has since
been moved to meson.build).

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@liaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>