s390/pci: prepare is_passed_through() for dma-iommu

s390/pci: prepare is_passed_through() for dma-iommu

With the IOMMU always controlled through the IOMMU driver testing for
zdev->s390_domain is not a valid indication of the device being
passed-through. Instead test if zdev->kzdev is set.

Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Link: https://lore.kernel.org/r/20230928-dma_iommu-v13-2-9e5fc4dacc36@linux.ibm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>