'Return size of an existing LV for manage_existing.
existing_ref is a dictionary of the form:
{\'name\': <name of existing volume on SF Cluster>}'

def manage_existing_get_size(self, volume, external_ref):

    sfid = external_ref.get('source-id', None)
    if (sfid is None):
        raise exception.SolidFireAPIException(_("Manage    existing    get    size    requires    'id'."))
    params = {'startVolumeID': int(sfid), 'limit': 1}
    vols = self._issue_api_request('ListActiveVolumes', params)['result']['volumes']
    return int(math.ceil((float(vols[0]['totalSize']) / units.Gi)))
