tests/functional/test_acpi_bits: Turn the test into a QemuSystemTest

tests/functional/test_acpi_bits: Turn the test into a QemuSystemTest

By using QemuSystemTest as a base class, we can use the set_machine()
command to check whether the required machine is available in the
binary (otherwise this test is failing when QEMU has been compiled
without the default 'pc' machine type).

Message-ID: <20241128115019.591362-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>