Currently lazy-pages daemon uses either pr->read_pages or get_remote_pages
to get actual page data from local images or remote server. From now on,
page_read will be completely responsible for getting the page data.

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>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>