Comment 5 for bug 1224444

> Is this a socket that libguestfs pre-creates on the host-side?

Yes it is:
https://github.com/libguestfs/libguestfs/blob/master/src/launch-direct.c#L208

You mention a scenario that might cause this. But that appears to be when the socket is opened. Note that the guest did send 4 bytes successfully (received OK at the host). The lost write occurs when the host next tries to send a message back to the guest.