dump/restore: Rework final-state switch

Remove CR_TASK_XXX states, use the TASK_XXX ones (for image). This is
required to unseize tasks properly in the next patches.

Plus, make sure that pstree_list and the seized set coincide (i.e.
handle error in collect_task).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>