src: report error from failing to add timer/FD watches

src: report error from failing to add timer/FD watches

The virEventAddHandle/Timeout APIs are unusual in that they do not
report errors on failure, because they call through to function
callbacks which might be provided externally to libvirt and thus
won't be using libvirt's error reporting APIs.

This is a rather unfortunate design characteristic as we can see
most callers forgot about this special behaviour and so we are
lacking error reporting in many cases.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>