s390x/pci: introduce S390PCIIOMMU

Currently each zpci device holds its own DMA address space and memory
region. At the same time, all instances of zpci device are stored in
S390pciState. So duirng the initialization of S390pciState, all zpci
devices are created and then all DMA address spaces are created. Thus,
when initializing pci devices, their corresponding DMA address spaces
could be found.

But zpci qdev will be introduced later. Zpci device may be initialized
and plugged afterwards generic pci device. So we should initialize all
DMA address spaces and memory regions before initializing zpci devices.

We introduce a new struct named S390PCIIOMMU. And a new field of
S390pciState, which is an array to store all instances of S390PCIIOMMU,
is added so that qemu pci code could find the corresponding DMA
address space when initializing a generic pci device. And this should
be done before the connection of a zpci device and a generic pci
device is built.

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