s390x/pci: Check memory region dispatching callbacks

The instructions PCI STORE, PCI LOAD and PCI STORE BLOCK
use calls to memory_region_dispatch_write() and
memory_region_dispatch_read() but do not test the return value.

Furthermore, the instruction PCI STORE BLOCK sets up a PGM_ADDRESSING
exception when the operand 3 is not within the designated PCI address
space instead of a PGM_OPERAND exception.

Let's setup a PGM_OPERAND exception in all of these failure cases.

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