net: add receive_disabled logic to iov delivery path

This patch adds the missing NetClient->receive_disabled logic in the
sendv delivery code path.  It seems that commit
893379efd0e1b84ceb0c42a713293f3dbd27b1bd ("net: disable receiving if
client returns zero") only added the logic to qemu_deliver_packet() and
not qemu_deliver_packet_iov().

The receive_disabled flag should be automatically set when .receive(),
.receive_raw(), or .receive_iov() return 0.  No further packets will be
delivered to the NetClient until the receive_disabled flag is cleared
again by calling qemu_flush_queued_packets().

Typically the NetClient will wait until its file descriptor becomes
writable and then invoke qemu_flush_queued_packets() to resume
transmission.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

net.c

@@ -423,16 +423,27 @@ ssize_t qemu_deliver_packet_iov(NetClientState *sender,
                                 void *opaque)
 {
     NetClientState *nc = opaque;
+    int ret;
 
     if (nc->link_down) {
         return iov_size(iov, iovcnt);
     }
 
+    if (nc->receive_disabled) {
+        return 0;
+    }
+
     if (nc->info->receive_iov) {
-        return nc->info->receive_iov(nc, iov, iovcnt);
+        ret = nc->info->receive_iov(nc, iov, iovcnt);
     } else {
-        return nc_sendv_compat(nc, iov, iovcnt);
+        ret = nc_sendv_compat(nc, iov, iovcnt);
     }
+
+    if (ret == 0) {
+        nc->receive_disabled = 1;
+    }
+
+    return ret;
 }
 
 ssize_t qemu_sendv_packet_async(NetClientState *sender,