[Libguestfs] nbdcpy: from scratch nbdcopy using io_uring

[Abhay Raj Singh]

Fixed the issue, I needed to learn to use Wireshark so took some time.
Turns out I was not updating the offset for an 'operation' / operation
slot in a rare case.

As an operation owns and NBD handle (the number) say 7

Initially, it read from offset 512 then 2048 then 4096, etc.
But was always writing to offset 512 (in rare cases) because I didn't
update the offset field of operation slot.

There's a bug where it gets stuck after reading 300KB or so, should be
done by tomorrow.

Thanks & Regards,
Abhay