That is not really necessary. Removing that node struct and put the
list entry directly into VTDAddressSpace. It simplfies the code a lot.
Since at it, rename the old notifiers_list into vtd_as_with_notifiers.
CC: QEMU Stable <qemu-stable@nongnu.org> Signed-off-by: Peter Xu <peterx@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>