If you are looking for the QEMU Guest Agent protocol reference, see:
/usr/share/doc/packages/qemu/interop/qemu-ga-ref.txt

The file you are reading is now provided simply to support a packaging
"Split-Provides", in order to maintain prior-release functionality. The
new package has split that functionality into another package which may
be optional going forward. If you find the package providing this file
is not needed, you may uninstall it.