migration: allow send_rq to fail

We will not allow failures to happen when sending data from destination
to source via the return path. However it is possible that there can be
errors along the way.  This patch allows the migrate_send_rp_message()
to return error when it happens, and further extended it to
migrate_send_rp_req_pages().

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180208103132.28452-9-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>