973387 - fix fsize attribute error on unit install progress reporting.

https://bugzilla.redhat.com/show_bug.cgi?id=973387

Cannot use super(self.__class__, self) because *DownloadBaseCallback* is not a subclass of *object*.

Apparently, the *fsize* attribute conditionally exists in urlgrabber.progress.BaseMeter (which sucks).  The DownloadBaseCallback.totsize attribute is safer to use but requires _do_start() to be updated.  The superclass method should be called in most cases anyway.