introduce lxcapi_add_device_node and lxcapi_remove_device_node to API (v3)

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

changes since v2;
* lets the user set an alternate path inside the container as Stéphane suggested

changes since v1;
* removed duplicated code

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>