s390x/pci: Fix memory_region_access_valid call

s390x/pci: Fix memory_region_access_valid call

In pcistb_service_handler, a call is made to validate that the memory
region can be accessed.  However, the call is made using the entire length
of the pcistb operation, which can be larger than the allowed memory
access size (8).  Since we already know that the provided buffer is a
multiple of 8, fix the call to memory_region_access_valid to iterate
over the memory region in the same way as the subsequent call to
memory_region_dispatch_write.

Fixes: 863f6f52b7 ("s390: implement pci instructions")
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Pierre Morel <pmorel@linux.ibm.com>
Message-Id: <1608243397-29428-3-git-send-email-mjrosato@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>