Clean-up: modernize pretty_xml.cpp

Since pretty_xml was converted to C++, clang-tidy had a number of
grievances such as the use of NULL (instead of nullptr).

Since the file is very small, it is modernized to address those issues:
  - Wrap libxml2 resources in RAII wrappers
  - Use C++ IO APIs in lieu of fprintf

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ie90e3e05130f7916f411e0de36e8aac72a0f790c