This patch introduces three helpers:
1) pstree_item_by_real() - search for pstree item by real pid.
2) pstree_item_by_virt() - search for pstree item by virtual pid.
3) pid_to_virt() - return virtual pis by real one.

Note: pstree_item_by_virt() and pid_to_virt() will be used to migrate AutoFS.
Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>