[libvirt] g_error considered harmful

[Christophe Fergeau]

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