[Libguestfs] [PATCH libnbd v4 0/2] copy: Use preferred block size for copying

v3 was: https://listman.redhat.com/archives/libguestfs/2022-June/029336.html v4 makes almost all of the recommended changes from Laszlo's reviews. I have dropped the third patch (the one which stablises copy/copy-nbd-error.sh test). That patch is still necessary, but it's not so urgent because the null: destination no longer advertises a 4M preferred block size, so the test fails only very rarely as before. Also I'd like to get Eric's input on that test when he's back. Rich.