Deprecate "process" in .pvx files

The "process" argument is now deprecated in .pvx files and support for it will be removed in a later release.

Examples have been updated accordingly.

A trivial .pvx file looks like this:

<?xml version="1.0" ?>
<pvx>
<Machine Name="localhost"
Environment="DISPLAY=:0"
Geometry="300x300+300+0"/>
</pvx>