This series was inspired by the discusion about invalid argument checking
in the public API and the unclear semantic of this functions.
https://www.redhat.com/archives/libvir-list/2011-May/msg01105.html
Clarify that the getter functions don't allow to obtain a subset of the
parameters. You can only get all at once.
Clarify that the setter functions can set a subset of the parameters.
Matthias