def vm_diskstats(vm_=None):
    '''
    Return disk usage counters used by the vms on this hyper in a
    list of dicts:

    .. code-block:: python

        [
            'your-vm': {
                'io_read_kbs'   : 0,
                'io_write_kbs'  : 0
                },
            ...
            ]

    If you pass a VM name in as an argument then it will return info
    for just the named VM, otherwise it will return all VMs.

    CLI Example:

    .. code-block:: bash

        salt '*' virt.vm_diskstats
    '''
    with _get_xapi_session() as xapi:
        def _info(vm_):
            ret = {}
            vm_uuid = _get_label_uuid(xapi, 'VM', vm_)
            if vm_uuid is False:
                return False
            for vbd in xapi.VM.get_VBDs(vm_uuid):
                vbd_rec = _get_record(xapi, 'VBD', vbd)
                ret[vbd_rec['device']] = _get_metrics_record(xapi, 'VBD',
                                                             vbd_rec)
                del ret[vbd_rec['device']]['last_updated']

            return ret

        info = {}
        if vm_:
            info[vm_] = _info(vm_)
        else:
            for vm_ in list_domains():
                info[vm_] = _info(vm_)
        return info