cgroup: treat devices as "special" properties

v2: don't try to write "" to devices.allow, just skip it since we write 'a'
    to devices.deny everywhere anyway.
v3: leave cgroup prop name as "devices.list" so it is caught in the later
    tests for special-ness. also, don't write the last devices entry twice.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>