Load virtuoso query engine (vsparql) in modular builds as well

This changeset completes 942bba27.  The virtuoso query engine (vsparql)
in a monolithic build is registered upon the STORAGE_VIRTUOSO condition
in librdf_init_query() whilst there was no equivalent procedure in the
modular case.

In this changeset, we hook into virtuoso's ltdl entry point routine
librdf_storage_module_register_factory() and register vsparql there
and then.  This will work as long as there's no additional initialiser
code in librdf_init_query() (which there isn't at the moment).