This PR implement the usage of fistpoint for testing some errors in smapi.
A fistpoint is an action that you can enable in the smapi to for example add a sleep at some point or raise an exception.
For example:

        with FistPoint(vm.host, "blktap_activate_inject_failure"):
            try:
                vm.start()
                vm.shutdown(force=True)
            except Exception as e:
                logging.info(f"Expected failure {e}")

Activating the fistpoint blktap_activate_inject_failure mean that the VDI activation will fail.
This fistpoint always raise an exception but most fistpoint just add a sleep in the code.
Using the fixture exit_on_fistpoint make all fistpoint raise an exception instead by enabling a special fistpoint called fist_LVHDRT_exit.