iohelper: Remove remaining newlines from error messages

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>