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)
lua: Don't redefine lua_objlen if already defined.

lua 5.2.2 seems to define this symbol already.

(cherry picked from commit 5ee193433a46d205e181bc51080e15f696bbda1a)