Documentation: userspace-api: iommufd: Update vDEVICE

Documentation: userspace-api: iommufd: Update vDEVICE

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

Link: https://patch.msgid.link/r/e1ff278b7163909b2641ae04ff364bb41d2a2a2e.1730836308.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>