Currently there're two functions, s390_pci_setup_msix() and
s390_pci_msix_init(), for msix initialization, and being called once
for each zpci device plugging. Let's integrate them.

Moreover msix is mandatory in s390 architecture. So we ensure the pci
device being plugged supports msix. For vfio (which is the only tested
setup so far), nothing changes.
Signed-off-by: NYi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: NPierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>