dump: Save and remove per-thread argumens in parasite

This patch starts using parasite_init_threads_seized and
parasite_fini_threads_seized helpers to save per-thread
data in parasite and remove it on cure procedure.

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