s390x/pci: fix failures of dma map/unmap

In commit d78c19b5cf4821d0c198f4132a085bdbf19dda4c, vfio code stores
the IOMMU's offset_within_address_space and adjusts the IOVA before
calling vfio_dma_map/vfio_dma_unmap. But s390_translate_iommu already
considers the base address of an IOMMU memory region.

Thus we use pal as the size and 0x0 as the base address to initialize
IOMMU memory subregion.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
