tests: Turn virFileWrapperAddPrefix to void

tests: Turn virFileWrapperAddPrefix to void

In theory, it's nice to have virFileWrapperAddPrefix() return a
value that indicates if the function succeeded or not. But in
practice, nobody checks for that and in fact blindly believes
that the function succeeded. Therefore, make the function return
nothing and just abort() if it would fail.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>