Bugfix for virtualpath

The virtual path was changed for all items without take in count the context. So you could change a url for a subpage "C" of a context by changing the virtualname of a parent page "A" of an other context (if the parent page "B" of the first context has the same virtual name than "A").

A simple example with 3 pages :
* A is in context main.
* B and C are in context second.
* A and B have the same virtual name "ok.html".
* C is the child of B (so it virtual path is "ok/C.html").
* You change A virtual name to "test.html".
* C virtual name become "test/C.html" instead of staying "ok/C.html".

This commit fix this.