Removes some auto-generated files still under version control.
It also moves the rule for generating NEWS into the Makefile.am
that's in the same directory as the output file to avoid confusion

* docs/libvirt-api.xml, docs/libvirt-refs.xml, NEWS: Remove
  auto-generated files from source control
* Makefile.am: Add rule for generating NEWS file
* docs/Makefile.am: Remove rule for generating NEWS file