page-xfer: remote-pages: allow receiving partial data

Since commit e609267f681062b4370e528a50f635222e0c2330 ("page-pipe: allow to
share pipes between page pipe buffers") the assumption that we will receive
the exact amount of pages we've requested with PS_IOV_GET does not always
hold.
In the case we serve pages data from the images using 'page-server
--lazy-page' the IOVs seen by the pagemap may cross page-pipe buffer
boundaries and read_page_pipe will clamp the pages in the response to those
boundaries.
Adjust page_server_read so it will not try to receive more pages than
page-server is going to send.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>