cfg2cmd/drive: don't use io_uring for krbd with wb/wt cache

cfg2cmd/drive: don't use io_uring for krbd with wb/wt cache

As reported here and locally reproduced:
https://forum.proxmox.com/threads/efi-vms-wont-start-under-7-beta-with-writeback-cache.91629/

This configuration is currently broken. Until we figure out how to fix
it properly, we can just have this (luckily very narrow) config pattern
fall back to aio=threads as it used to.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>