gconfig: Fix leak in gvir_config_domain_filesys_set_ram_usage
gconfig: Fix leak in gvir_config_domain_filesys_set_ram_usage

The object returned by gvir_config_object_replace_child() must be
unref'ed when no longer needed.

libvirt/libvirt-glib,libvirt/libvirt-glib,libvirt/libvirt-glib