qdev: Fix qdev_try_create() semantics

Since QOM'ification, qdev_try_create() uses object_new() internally,
which asserts "type != NULL" when the type is not registered.
This was revealed by the combination of kvmclock's kvm_enabled() check
and early QOM type registration.

Check whether the class exists before calling object_new(), so that
the caller (e.g., qdev_create) can fail gracefully, telling us which
device could not be created.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Cc: Anthony Liguori <aliguori@codemonkey.ws>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/qdev.c

@@ -117,6 +117,9 @@ DeviceState *qdev_try_create(BusState *bus, const char *name)
 {
     DeviceState *dev;
 
+    if (object_class_by_name(name) == NULL) {
+        return NULL;
+    }
     dev = DEVICE(object_new(name));
     if (!dev) {
         return NULL;