For asynchrounous page transfers in post-copy migration we need to be able
to request a remote pages, receive back information about the data is going
to arrive and receive the page data itself.

travis-ci: success for uffd: A new set of improvements
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>