hw/qdev: Don't crash if qdev_create(NULL, ...) fails

If an attempt to create a qdev device on the default sysbus (by passing
NULL as the bus to qdev_create) fails, print a useful error message
rather than crashing trying to dereference a NULL pointer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

hw/qdev.c

@@ -111,7 +111,12 @@ DeviceState *qdev_create(BusState *bus, const char *name)
 
     dev = qdev_try_create(bus, name);
     if (!dev) {
-        hw_error("Unknown device '%s' for bus '%s'\n", name, bus->info->name);
+        if (bus) {
+            hw_error("Unknown device '%s' for bus '%s'\n", name,
+                     bus->info->name);
+        } else {
+            hw_error("Unknown device '%s' for default sysbus\n", name);
+        }
     }
 
     return dev;