src: make virObjectUnref return void

src: make virObjectUnref return void

To prepare for a conversion to GObject, we need virObjectUnref
to have the same API design as g_object_unref, which means it
needs to be void.

A few places do actually care about the return value though,
and in these cases a thread local flag is used to determine
if the dispose method was invoked.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>