intel_iommu: enable remote IOTLB

This patch is based on Aviv Ben-David (<bd.aviv@gmail.com>)'s patch
upstream:

  "IOMMU: enable intel_iommu map and unmap notifiers"
  https://lists.gnu.org/archive/html/qemu-devel/2016-11/msg01453.html

However I removed/fixed some content, and added my own codes.

Instead of translate() every page for iotlb invalidations (which is
slower), we walk the pages when needed and notify in a hook function.

This patch enables vfio devices for VT-d emulation.

And, since we already have vhost DMAR support via device-iotlb, a
natural benefit that this patch brings is that vt-d enabled vhost can
live even without ATS capability now. Though more tests are needed.

Signed-off-by: Aviv Ben-David <bdaviv@cs.technion.ac.il>
Reviewed-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: \"Michael S. Tsirkin\" <mst@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1491562755-23867-10-git-send-email-peterx@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
