spec/vhost-user spec: Add IOMMU support

spec/vhost-user spec: Add IOMMU support

This patch specifies and implements the master/slave communication
to support device IOTLB in slave.

The vhost_iotlb_msg structure introduced for kernel backends is
re-used, making the design close between the two backends.

An exception is the use of the secondary channel to enable the
slave to send IOTLB miss requests to the master.

Signed-off-by: Maxime Coquelin <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>