iommufd: Use enum iommu_viommu_type for type in struct iommufd_viommu

iommufd: Use enum iommu_viommu_type for type in struct iommufd_viommu

Replace unsigned int, to make it clear. No functional changes.

The viommu_alloc iommu op will be deprecated, so don't change that.

Link: https://patch.msgid.link/r/6c6ba5c0cd381594f17ae74355872d78d7a022c0.1749882255.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>