restore: allocate pstree items from a shared pool (v2)

A pstree item will contain pid and state. Both these properties will be
determined after creating processes and will be used from the
controlling proces.

v2: fix the error path
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>