nit fix for _cleanOldFiles

svdsm.sock will not be in filterd fileType

Change-Id: I8d37c524c1a2ad7aadf36a27f524c378f6a104d3
Signed-off-by: Royce Lv<lvroyce@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/11128
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>