The page-pipe is an object, that can accumulate pages inside it. It
consists of list of page-pipe-bufs, which in turn has a pipa, an
array of iovecs that describe the pages' locations and some stats.

Users of it are supposed to vmsplice pages into pipes to accumulate
then for later use, and vmsplice them from pipes when required.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>