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>