s390x/pci: use a PCI Group structure

s390x/pci: use a PCI Group structure

We use a S390PCIGroup structure to hold the information related to a
zPCI Function group.

This allows us to be ready to support multiple groups and to retrieve
the group information from the host.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>