s390x/pci: fix pcistb length

In pcistb_service_call, we are grabbing 8 bits from a guest register to
indicate the length of the store operation -- but per the architecture
the length is actually defined by 13 bits of the guest register.

Fixes: 863f6f52 ("s390: implement pci instructions")
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <1608243397-29428-2-git-send-email-mjrosato@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>