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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 generate_cpu_xml(self):
 """ Get CPU XML Returns: lxml.etree.Element: cpu node """

        if self.cpu_custom:
            return self.generate_custom(
                cpu=self.cpu,
                vcpu_num=self.vcpu_num,
                fill_topology=self.vcpu_set
            )
        elif self.cpu_model:
            return self.generate_exact(
                self.cpu_model, vcpu_num=self.vcpu_num, host_cpu=self.host_cpu
            )
        else:
            return self.generate_host_passthrough(self.vcpu_num)