Add to plugin-tutorial.tex on virtualizing ids

A wrapper can be called from the target app (receiving a virtual id)
or from another function iwthin the library (receiving a real id).
It must be prepared to distinguish the two cases in this situation.


git-svn-id: svn://svn.code.sf.net/p/dmtcp/code/trunk@2277 ba6dc0ae-232f-0410-883f-9ebac64fc565