ui: qemu disk: honor iothread setting from config

ui: qemu disk: honor iothread setting from config

To have a IOThread on by default in the wizard and on disk add, we
added a 'bind' for the value here. This also changes the value for
existing VM disks, and if one does not notice, modifies it away again
(since we don't have the controller here and isSCSISingle is false)

Simply don't bind value when we edit a VM disk from config

Note that this is only an issue in Chromium based browsers.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>