Ignore any container with a name starting by '.'

Ignore any container with a name starting by '.'

  * This are either '.', '..' or a hidden directory.
    And this names should not be used for a container
    in any case.

  * Before this patch, if you created a git repository under lxc.lxcpath (it
    can be useful to keep track of the configurations of your containers)

    Then, when you run lxc-ls you will get the following output:

        # lxc-ls
        .git      container1      container2    ....

    This is because there is a 'config' file inside the '.git' directory.
    It is where git stores the configuration of the repository.

    And the test lxc-ls does to check if a directory contains a container
    is just to check if the 'directory/config' file exists.

Signed-off-by: Carlos Alberto Lopez Perez <clopez@igalia.com>