Description of problem:
Upstream NBD has finally accepted a spec change for 64-bit extensions (still listed as experimental until there are enough implementations; but BZ 2182208 tracks implementing the same extensions in qemu). Libnbd has a proof-of-concept implementation that now needs to be polished and incorporated.
https://listman.redhat.com/archives/libguestfs/2022-November/030316.html

Version-Release number of selected component (if applicable):
v1.16

How reproducible:
100%

Steps to Reproduce:
1. Create a 5G image, serve it over qemu-nbd (or qemu-storage-daemon) patched for 64-bit extensions, then use libnbd to connect to it and issue a 64-bit write zero or block status command.
3.

Actual results:
Without extension support, libnbd will flag the request as being oversized

Expected results:
With the extension active in both client and server, requests larger than 32-bit effect length should be possible

Additional info: