pipes: Make dump_one_pipe_data/restore_pipe_data being non-static

To reuse with fifo data restore.

v2:
 - use image type in functions arguments instead of forcing
   calling side to provide file descriptor

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