qom: introduce root device

This is based on Jan's suggestion for how to do unique naming.  The root device
is the root of composition.  All devices are reachable via child<> links from
this device.
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

Makefile.objs

@@ -278,7 +278,7 @@ hw-obj-$(CONFIG_LSI_SCSI_PCI) += lsi53c895a.o
 hw-obj-$(CONFIG_ESP) += esp.o
 
 hw-obj-y += dma-helpers.o sysbus.o isa-bus.o
-hw-obj-y += qdev-addr.o
+hw-obj-y += qdev-addr.o container.o
 
 # VGA
 hw-obj-$(CONFIG_VGA_PCI) += vga-pci.o

hw/container.c

+#include "sysbus.h"
+
+static int container_initfn(SysBusDevice *dev)
+{
+    return 0;
+}
+
+static SysBusDeviceInfo container_info = {
+    .init = container_initfn,
+    .qdev.name = "container",
+    .qdev.size = sizeof(SysBusDevice),
+    .qdev.no_user = 1,
+};
+
+static void container_init(void)
+{
+    sysbus_register_withprop(&container_info);
+}
+
+device_init(container_init);

hw/qdev.c

@@ -1161,3 +1161,15 @@ void qdev_property_add_legacy(DeviceState *dev, Property *prop,
 
     g_free(type);
 }
+
+DeviceState *qdev_get_root(void)
+{
+    static DeviceState *qdev_root;
+
+    if (!qdev_root) {
+        qdev_root = qdev_create(NULL, "container");
+        qdev_init_nofail(qdev_root);
+    }
+
+    return qdev_root;
+}

hw/qdev.h

@@ -482,4 +482,12 @@ const char *qdev_property_get_type(DeviceState *dev, const char *name,
  */
 void qdev_property_add_legacy(DeviceState *dev, Property *prop, Error **errp);
 
+/**
+ * @qdev_get_root - returns the root device of the composition tree
+ *
+ * Returns:
+ *   The root of the composition tree.
+ */
+DeviceState *qdev_get_root(void);
+
 #endif