BUG-2511: disable external entitiy resolution with EXI

OpenEXI transmogrifier can resolve external entities. This should never
be necessary in NETCONF context, so disallow that. The same goes for the
reader, but there we need to add a custom EntityResolver.

Origial work done by Robert Vargag. Cherry-picked from
8350f87e0e1190fcc84099d3b7446a2f1de9e5f9 by Colin Dixon.

Change-Id: Idf4b7faf13063c88624f2ba16f3871679c072b3e
Signed-off-by: Robert Varga <rovarga@cisco.com>
Signed-off-by: Colin Dixon <colin@colindixon.com>