qom: Refine container_get() to allow using a custom root

Specify the root to search from as argument. This avoids hardcoding
"/machine" in some places and makes it more flexible.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Anthony Liguori <anthony@codemonkey.ws>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/qdev.c b/hw/qdev.c
index afbc975..0bcde20 100644
--- a/hw/qdev.c
+++ b/hw/qdev.c
@@ -158,8 +158,9 @@
         static int unattached_count = 0;
         gchar *name = g_strdup_printf("device[%d]", unattached_count++);
 
-        object_property_add_child(container_get("/machine/unattached"), name,
-                                  OBJECT(dev), NULL);
+        object_property_add_child(container_get(qdev_get_machine(),
+                                                "/unattached"),
+                                  name, OBJECT(dev), NULL);
         g_free(name);
     }
 
@@ -677,7 +678,7 @@
     static Object *dev;
 
     if (dev == NULL) {
-        dev = container_get("/machine");
+        dev = container_get(object_get_root(), "/machine");
     }
 
     return dev;