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: NPeter Xu <peterx@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit b4a4ba0d)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>