apparmor: add attach_disconnected

apparmor: add attach_disconnected

Otherwise we fail to reconnect to /dev/net/tun opened by libvirtd
like

    [ 8144.507756] audit: type=1400 audit(1505488162.386:38069121): apparmor="DENIED" operation="file_perm" info="Failed name lookup - disconnected path" error=-13 profile="libvirt-5dfcc8a7-b79a-4fa9-a41f-f6271651934c" name="dev/net/tun" pid=9607 comm="qemu-system-x86" requested_mask="r" denied_mask="r" fsuid=117 ouid=0

Reviewed-By: Jamie Strandboge <jamie@canonical.com>
Acked-By: Michal Privoznik <mprivozn@redhat.com>