pagemap: add ability to request remote pages

The asynchronous version of remote page_read will send the request to the
dump side and return happily.
The response will be handled by the uffd.c because it's epoll loop is the
only place where we can handle events.

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>