iommufd/viommu: Explicitly define vdev->virt_id

iommufd/viommu: Explicitly define vdev->virt_id

The "id" is too general to get its meaning easily. Rename it explicitly to
"virt_id" and update the kdocs for readability. No functional changes.

Link: https://patch.msgid.link/r/1fac22d645e6ee555675726faf3798a68315b044.1752126748.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>