criu: lazy-pages: enable remoting of lazy pages

The remote lazy pages variant can be run as follows:

src# criu dump -t <pid> --lazy-pages --port 9876 -D /tmp/1 &
src# while ! sudo fuser 9876/tcp ; do sleep 1; done
src# scp -r /tmp/1/ dst:/tmp/

dst# criu lazy-pages --page-server --address dst --port 9876 -D /tmp/1 &
dst# criu restore --lazy-pages -D /tmp/1

In a nutshell, this implementation of remote lazy pages does the following:

- dump collects the process memory into the pipes, transfers non-lazy pages
  to the images or to the page-server on the restore side. The lazy pages
  are kept in pipes for later transfer
- when the dump creates the page_pipe_bufs, it marks the buffers containing
potentially lazy pages with PPB_LAZY
- at the dump_finish stage, the dump side starts TCP server that will
handle page requests from the restore side
- the checkpoint directory is transferred to the restore side
- on the restore side lazy-pages daemon is started, it creates UNIX socket
to receive uffd's from the restore and a TCP socket to forward page
requests to the dump side
- restore creates memory mappings and fills the VMAs that cannot be handled
by uffd with the contents of the pages*img.
- restore registers lazy VMAs with uffd and sends the userfault file
descriptors to the lazy-pages daemon
- when a #PF occurs, the lazy-pages daemon sends PS_IOV_GET command to the dump
side; the command contains PID, the faulting address and amount of pages
(always 1 at the moment)
- the dump side extracts the requested pages from the pipe and splices them
into the TCP socket.
- the lazy-pages daemon copies the received pages into the restored process
address space

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>