glusterd: Add xml specific functions in HAVE_LIB_XML block

Build failure on OSX and also on Linux with '--disable-xml-output'
introduced in following commit "c080403393987f807b9ca81be140618fa5e994f1"

Change-Id: I0db92c9f5e319dc1932bed9ecc1acc98adb57de3
BUG: 983317
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/8546
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>