mem: Drain memory from parasite before dumping it into file

Currently we dump pages directly from parasite into image files. This
is bad for several reasons:

1. We cannot use any more-or-less custom format for pages easily, since
   parasite code cannot be linked with any libraries;
2. We will not be able to optimize migration with preliminary memory
   migration (a.k.a. iterative migration) with it -- if we send pages
   from parasite over network we are not able to let the task we dump
   continue running.

That said, what is done is -- pages from target task are put into a
page-pipe in one go, then (not in this patch) parasite can be released
and we can do with pages whatever we want. For now pages are just
spliced from pipe into image file.

Some numbers:
In order to drain 1Gb of memory from task we need 1.5M of shared map
in args (for iovecs) and 4 pipes (8 descriptors) each referencing 128Mb
of pages, which int turn requires 4 x 640K chunks of sequential kernel
memory (for pipe_buffer). Not that big I guess.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>