The synchronous remote page transfer prevents reception of uffd events
during the communications with the page server on the dump side. Adding
socket file descriptor to epoll_wait allows processing of incoming uffd
events after non-blocking request for remote page is issued and before the
dump side page server replies.

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>