[libvirt] g_error considered harmful

Hi, g_error calls abort() when it's called, thus it should only be used for bad assertion failures. We currently are using it in some places in libvirt-gobject as if it was g_warning. This patch series gets rid of most occurrences of g_error by either replacing it with g_warning or by adding a GError ** argument to the function calling g_error. Christophe