LXC: Creating devices for container on host side

LXC: Creating devices for container on host side

user namespace doesn't allow to create devices in
uninit userns. We should create devices on host side.

We first mount tmpfs on dev directroy under state dir
of container. then create devices under this dev dir.

Finally in container, mount the dev directroy created
on host to the /dev/ directroy of container.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>