s390x/pci: handle PCIBridge bus number

s390x/pci: handle PCIBridge bus number

The PCI bus number is usually set by the host during the enumeration.

In the s390 architecture we neither get a Device Tree nor have an
enumeration understanding bridge devices.

Let's fake the enumeration on reset and set the PCI_PRIMARY_BUS,
PCI_SECONDARY_BUS and PCI_SUBORDINATE_BUS config entries for the
bridges.

Let's add the configuration of these three config entries on bridge hot
plug.

The bus number is calculated based on a new entry, bus_num of the
S390pciState device.

This commit is inspired by what spapr pci does.

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