Properly initialize libxml2

When libxml2 is to be used in a multi-threaded application, the
xmlInitThreads() function must be called before any other libxml2
function.  This function does different things on various platforms and
thus one can get away without calling it on Unix systems, but not on
Windows, where it initializes critical section objects used for
synchronizing access to data structures shared between threads.  Add the
missing xmlInitThreads() call to prevent crashes on affected systems.

Also add a matching xmlCleanupThreads() call to properly release the
resources set up by xmlInitThreads().