storage: fix crash when listing volumes or undefining a pool
    
    The commit cad3cf9a951d26da9d2ee0f5b52fb1a2dbb74af1 introduced a crash
    due to wrong order of parameters being passed to the function.  When
    deleting an element, the function decreased the iterator instead of
    count and if listing volumes after that (or undefining the pool, NULL
    was being dereferenced.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>