qdev-monitor: Fix crash when device_add is called with abstract driver

User is able to crash running QEMU when following monitor
command is called:

 device_add intel-hda-generic

Crash is caused by assertion in object_initialize_with_type()
when type is abstract.

Checking if type is abstract before instance is created in
qdev_device_add() allows to prevent crash on incorrect user input.

Cc: qemu-stable@nongnu.org
Signed-off-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>

qdev-monitor.c

@@ -481,6 +481,12 @@ DeviceState *qdev_device_add(QemuOpts *opts)
         return NULL;
     }
 
+    if (object_class_is_abstract(oc)) {
+        qerror_report(QERR_INVALID_PARAMETER_VALUE, "driver",
+                      "non-abstract device type");
+        return NULL;
+    }
+
     dc = DEVICE_CLASS(oc);
 
     /* find bus */