page-xfer: Don't expect that page server cmd is read in one call

There's a standard way of doing this -- the MSG_WAITALL flag that
works even for TCP sockets.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

page-xfer.c

@@ -167,7 +167,7 @@ static int page_server_serve(int sk)
 	while (1) {
 		struct page_server_iov pi;
 
-		ret = read(sk, &pi, sizeof(pi));
+		ret = recv(sk, &pi, sizeof(pi), MSG_WAITALL);
 		if (!ret)
 			break;