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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def guest_start(self, userid):
 """Power on a virtual machine. :param str userid: the id of the virtual machine to be power on :returns: None """

        action = "start guest '%s'" % userid
        with zvmutils.log_and_reraise_sdkbase_error(action):
            self._vmops.guest_start(userid)