Qemu: How to create the bridge device, bus, and pci device.

How to create bridge device, bus, and pci device.

As we all know, the "device and bus" architecture in qemu is that, a pci
device should attach to a pci bus, and a pci bus should add to a bridge
device, and the bridge device should link to main-system-bus.

To realize this, four important functions are introduced:
1. qdev_bridge_create() to create a bridge device named "s390-bridge-dev".
2. qbus_create() to create a pci bus named "s390-pci-bus" and attach
   "s390-pci-bus" to "s390-bridge-dev".
3. s390_pci_bus_init() is used by coder to assign a bridge device and a pci bus,
   it will call qdev_bridge_create() and qbus_create().
4. qdev_device_add() to create a pci device and attach it to designated bus.

The following codes are telling how to realize this in detail.

I. Define TypeInfo in test_pci.c file.

static void s390_pci_dev_class_init(ObjectClass *kclass, void *data)
{
    DeviceClass *dc = DEVICE_CLASS(kclass);

    dc->bus_type = TYPE_S390_PCI_BUS;
}

static const TypeInfo s390_bridge_dev_info = {
    .name       = TYPE_S390_BRIDGE_DEV,
    .parent     = TYPE_SYS_BUS_DEVICE,
    .class_init = s390_bridge_dev_class_init,
}

static const TypeInfo s390_pci_bus_info = {
    .name       = TYPE_S390_PCI_BUS,
    .parent     = TYPE_BUS,
}

static const TypeInfo s390_pci_dev_info = {
    .name       = TYPE_S390_PCI_DEV,
    .parent     = TYPE_DEVICE,
    .class_init = s390_pci_dev_class_init,
};

II. Define macro in test_pci.h file.

#define TYPE_S390_BRIDGE_DEV  "s390-bridge-dev"
#d