This introduces a new add_devices() call to the python API.

Parameters:
 - path => Mandatory, path to a character or block device on the host
 - destpath => Optional, alternative path inside the container

The function will allow the node in the container's devices cgroup and
then create the entry in the container.
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>