confile: sanity-check netdev->type before setting netdev->priv elements

The netdev->priv is shared for the netdev types.  A bad config file
could mix configuration for different types, resulting in a bad
netdev->priv when starting or even destroying a container.  So sanity
check the netdev->type before setting a netdev->priv element.

This should fix https://github.com/lxc/lxc/issues/254

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>