Documentation: userspace-api: iommufd: Update vIOMMU

Documentation: userspace-api: iommufd: Update vIOMMU

With the introduction of the new object and its infrastructure, update the
doc to reflect that and add a new graph.

Link: https://patch.msgid.link/r/7e4302064e0d02137c1b1e139342affc0485ed3f.1730836219.git.nicolinc@nvidia.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>