autodev: switch strategies (v3)

autodev: switch strategies (v3)

Do not keep container devs under /dev/.lxc.  Instead, always
keep them in a small tmpfs mounted at $(mounted_root)/dev.

The tmpfs is mounted in the container monitor's namespace.  This
means that at every reboot it will get re-created.  It seems to
me this better replicates what happens on a real host.

If we want devices persisting across reboots, then perhaps we can
implement a $lxcpath/$name/keepdev directory containing devices to
bind into the container at each startup.

Changelog (v2): don't bother with the $lxcpath/$name/rootfs.dev
	directory, just mount the tmpfs straight into the container.

Changelog (v3): Don't create /dev if it doesn't exist

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