tools: nfd-status-http-server: serve XML

refs #1690

Change-Id: Ie5de0b8e5041f91cf9bdf1ec7e2c24cde5ffbe38
tools: Corrections and visual improvements for nfd-status-http-server

Change-Id: I123020f023d9dc52268a35d4ad7aa4752e79309b
Refs: #1690