introduce lxcapi_add_device_node and lxcapi_remove_device_node to API

Adding block/char devices to running container is a common operation so provide a common
implementation for users to consume.

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>