core: fix CVE-2014-3573

single place in which DocumentBuilderFactory is constructed to apply
security settings.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1139000
Change-Id: Icf27db1ec13b6a16d9b7c77fd9710e8e6f6ec3c9
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>