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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 list_default_vsan_policy(service_instance=None):
    '''
    Returns the default vsan storage policy.

    service_instance
        Service instance (vim.ServiceInstance) of the vCenter.
        Default is None.

    .. code-block:: bash

        salt '*' vsphere.list_storage_policies

        salt '*' vsphere.list_storage_policy policy_names=[policy_name]
    '''
    profile_manager = salt.utils.pbm.get_profile_manager(service_instance)
    policies = salt.utils.pbm.get_storage_policies(profile_manager,
                                                   get_all_policies=True)
    def_policies = [p for p in policies
                    if p.systemCreatedProfileType == 'VsanDefaultProfile']
    if not def_policies:
        raise VMwareObjectRetrievalError('Default VSAN policy was not '
                                         'retrieved')
    return _get_policy_dict(def_policies[0])