'Checks an existing initiator group in the masking view.
Check if the initiators in the initiator group match those in the
system.
:param serial_number: the array serial number
:param maskingview_name: name of the masking view
:param masking_view_dict: masking view dict
:param storagegroup_name: the storage group name
:param portgroup_name: the port group name
:param extra_specs: the extra specifications
:returns: ig_from_mv, msg'

def _check_existing_initiator_group(self, serial_number, maskingview_name, masking_view_dict, storagegroup_name, portgroup_name, extra_specs):

    msg = None
    ig_from_mv = self.rest.get_element_from_masking_view(serial_number, maskingview_name, host=True)
    check_ig = masking_view_dict[utils.INITIATOR_CHECK]
    if check_ig:
        (check, found_ig) = self._verify_initiator_group_from_masking_view(serial_number, maskingview_name, masking_view_dict, ig_from_mv, storagegroup_name, portgroup_name, extra_specs)
        if (not check):
            msg = ('Unable    to    verify    initiator    group:    %(ig_name)s    in    masking    view    %(maskingview_name)s.' % {'ig_name': ig_from_mv, 'maskingview_name': maskingview_name})
            LOG.error(msg)
    return (ig_from_mv, msg)
