The page transfer protocol is completely synchronous on the dump side,
therefore we can presume that when we get POLLIN event on the page server
socket it is either page info response for the last sent page request or
the page data following the last page info.
In the first case we set ev_data associated with page server socket events
to values received in receive_remote_page_info and in the second case we
reset ev_data to zero. This allows us to distinguish what was the reason
page_server_event have been called.

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>