s390x/pci : fix up s390 pci iommu translation function

On s390x, each pci device has its own iommu, which is only properly
setup in qemu once the mpcifc instruction used to register the
translation table has been intercepted. Therefore, for a pci device that
is not configured or has not been initialized, proper translation is
neither required nor possible. Moreover, we may not have a host bridge
device ready yet.

This was exposed by a recent vfio change that triggers iommu translation
during the initialization of the vfio pci device. Let's do an early exit
in that case.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
