qdev-monitor-test: add device_add leak test cases

Ensure that the device_add error code path deletes device objects.
Failure to do so not only leaks the objects but can also keep other
objects (like drive or netdev) alive due to qdev properties holding
references.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
