iommufd/viommu: Add iommufd_viommu_get_vdev_id helper

iommufd/viommu: Add iommufd_viommu_get_vdev_id helper

This is a reverse search v.s. iommufd_viommu_find_dev, as drivers may want
to convert a struct device pointer (physical) to its virtual device ID for
an event injection to the user space VM.

Again, this avoids exposing more core structures to the drivers, than the
iommufd_viommu alone.

Link: https://patch.msgid.link/r/18b8e8bc1b8104d43b205d21602c036fd0804e56.1741719725.git.nicolinc@nvidia.com
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>