'Return size of volume to be managed by manage_existing.
existing_ref is a dictionary of the form:
{\'source-name\': <name of the virtual volume>}'

@cinder_utils.trace
def manage_existing_get_size(self, volume, existing_ref):

    self._check_api_version()
    target_vol_name = self._get_existing_volume_ref_name(existing_ref)
    if re.match('volume-*|snapshot-*', target_vol_name):
        reason = _('Reference    must    be    the    volume    name    of    an    unmanaged    virtual    volume.')
        raise exception.ManageExistingInvalidReference(existing_ref=target_vol_name, reason=reason)
    client = self._login()
    try:
        volume_info = client.getVolumeByName(target_vol_name)
    except hpeexceptions.HTTPNotFound:
        err = (_("Virtual    volume    '%s'    doesn't    exist    on    array.") % target_vol_name)
        LOG.error(err)
        raise exception.InvalidInput(reason=err)
    finally:
        self._logout(client)
    return int(math.ceil((float(volume_info['size']) / units.Gi)))
