cg: restore special cpuset properties

cpuset.cpus and cpuset.mems can't be written to for the first time after they
have tasks, so the traditional mechanism of restoring properties after
restoring the tasks won't work here. Instead, we copy the parent values of the
properties into them, restore the tasks, and then restore via the traditional
mechanism the actual values of these properties.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>