iohelper: Remove remaining newlines from error messages
    
    The iohelper is an internal program that's only supposed to
    be called by libvirt, and whatever output it might produce
    will ultimately be passed to virReportError() or similar.
    
    Since we do not want strings passed to those functions to
    contain newlines, we can simply not output them in the first
    place.
    
    This is what happens in pretty much all cases already, but
    in a couple instances newlines have managed to slip in.
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>
    Reviewed-by: Laine Stump <laine@laine.org>