:param prefix: sd|vd|xvd

def unix_device(self, prefix=None):
        """
        :param prefix: sd|vd|xvd
        """
        prefix = prefix or self.random_element(self.unix_device_prefixes)
        suffix = self.random_element(string.ascii_lowercase)
        path = '/dev/%s%s' % (prefix, suffix)
        return path