rewrite iov_send_recv() and move it to iov.c

Make it much more understandable, add a missing
iov_cnt argument (number of iovs in the iov), and
add comments to it.

The new implementation has been extensively tested
by splitting a large buffer into many small
randomly-sized chunks, sending it over socket to
another, slow process and verifying the receiving
data is the same.

Also add a unit test for iov_send_recv(), sending/
receiving data between two processes over a socketpair
using random vectors and random sizes.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

qemu-coroutine-io.c

@@ -34,7 +34,7 @@ qemu_co_sendv_recvv(int sockfd, struct iovec *iov, unsigned iov_cnt,
     size_t done = 0;
     ssize_t ret;
     while (done < bytes) {
-        ret = iov_send_recv(sockfd, iov,
+        ret = iov_send_recv(sockfd, iov, iov_cnt,
                             offset + done, bytes - done, do_send);
         if (ret > 0) {
             done += ret;