vhost_net: device IOTLB support

This patches implements Device IOTLB support for vhost kernel. This is
done through:

1) switch to use dma helpers when map/unmap vrings from vhost codes
2) introduce a set of VhostOps to:
   - setting up device IOTLB request callback
   - processing device IOTLB request
   - processing device IOTLB invalidation
2) kernel support for Device IOTLB API:

- allow vhost-net to query the IOMMU IOTLB entry through eventfd
- enable the ability for qemu to update a specified mapping of vhost
- through ioctl.
- enable the ability to invalidate a specified range of iova for the
  device IOTLB of vhost through ioctl. In x86/intel_iommu case this is
  triggered through iommu memory region notifier from device IOTLB
  invalidation descriptor processing routine.

With all the above, kernel vhost_net can co-operate with userspace
IOMMU. For vhost-user, the support could be easily done on top by
implementing the VhostOps.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
