lxc_start: ERROR if container is already running.

We should exit with a error when starting a running container.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
container: introduce two functions named as {at/de}tach_interface().

Currently, we depends on ip command to attach interface to container.
It means we only implemented it by python.

This patch implement adding and removing interface by c and added
them in struct container.

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>