[trace] fix on link type creation (was indexed by type name, but should consider...

[trace] fix on link type creation (was indexed by type name, but should consider source and dest)

details:
- small re-organization of the code, type declaration is made on get function
if the type the caller is looking for is not found

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/simgrid/simgrid/trunk@9303 48e7efb5-ca39-0410-a469-dd3cf9ba447f