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.

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