lib: xmlParse{File,Memory} -> xmlRead{File,Memory}

Switch from xmlParse* to xmlRead* so we can explicitly specify parsing
flags, including no network usage and no entity resolution.
The two behaviours mentioned above were not done before either, so this
should not introduce behaviour changes in libguestfs.

(cherry picked from commit 845daded5fddc70fc5e822769bc1e2a8cbead7ca)