s390x/pci: rework PCI STORE BLOCK

Enhance the fault detection.

Fixup the precedence to check the destination path existance
before checking for the source accessibility.

Add the maxstbl entry to both the Query PCI Function Group
response and the PCIBusDevice structure.

Initialize the maxstbl to 128 per default until we get
the actual data from the hardware.

Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Reviewed-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Message-Id: <1512046530-17773-5-git-send-email-pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
