intel-iommu: remove IntelIOMMUNotifierNode

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>