Files listed as _DATA in Makefile.am are installed without the
executable bit set, files listed as _SCRIPT do get the executable bit
set. As spice_codegen.py is meant to be run by spice-protocol users,
make sure this bit is set.
The just added .proto files and python code generation code need to be
added to EXTRA_DIST in order to be distributed in tarballs, and
we need to recurse in the new python_modules directory.
