Iperf and vnetif improvements

[MagnusS]

• Remove unnecessary calls to sleep
• Remove calls to flush when writing to pcap output channel
• Catch pcap write errors

[MagnusS]

The listener function that writes to the pcap file is called with Lwt.async by the backend, resulting in a race when closing the pcap output channel. If packets are written to the channel while it is being closed this triggers a Channel_closed exception and the test fails. There is still a race with this patch, but the exception is caught. I have also removed the unnecessary call to Lwt_io.flush which should make it more likely that the recorder has completed before the channel is closed.

[samoht]

Thanks!