hw: Make MachineClass::is_default a boolean type

hw: Make MachineClass::is_default a boolean type

There's no good reason for it to be type int, change it to bool.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200207161948.15972-3-philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>