Re-arrange python generator to make it clear what's auto-generated

Re-arrange python generator to make it clear what's auto-generated

* README: New file describing what each file is used for
* livvirt-override.c, libvirt-override.py, libvirt-override-api.xml,
  libvirt-override-virConnect.py: Manually written code overriding
  the generator
* typewrappers.c, typewrappers.h: Data type wrappers
* generator.py: Automatically pre-prend contents of libvirt-override.py
  to generated libvirt.py. Output into libvirt.py directly instead of
  libvirtclass.py. Don't generate libvirtclass.txt at all. Write C
  files into libvirt.c/.h directly
* Makefile.am: Remove rule for creating libvirt.py from libvirt-override.py
  and libvirtclass.py, since generator.py does it directly