virHashAddEntry: Report error on duplicate key
    
    If this function fails, the error message is reported only in
    some cases (e.g. OOM), but in some it's not (e.g. duplicate key).
    This fact is painful and we should either not report error at all
    or report the error in all possible cases. I vote for the latter.
    Unfortunately, since the key may be an arbitrary value (not
    necessarily a string) we can't report it in the error message.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>