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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 vbd_list(name=None, call=None):
    '''
    Get a list of VBDs on a VM

    **requires**: the name of the vm with the vbd definition

    .. code-block:: bash

        salt-cloud -a vbd_list xenvm01

    '''
    if call == 'function':
        raise SaltCloudSystemExit(
            'This function must be called with -a, --action argument.'
        )
    if name is None:
        return 'A name kwarg is rquired'
    ret = {}
    data = {}
    session = _get_session()
    vms = session.xenapi.VM.get_by_name_label(name)
    if len(vms) == 1:
        vm = vms[0]
        vbds = session.xenapi.VM.get_VBDs(vm)
        if vbds is not None:
            x = 0
            for vbd in vbds:
                vbd_record = session.xenapi.VBD.get_record(vbd)
                data['vbd-{}'.format(x)] = vbd_record
                x += 1
    ret = data
    return ret