default IOTHREAD no applied when attaching a virtio disk

[atodorov-storpool]

Description
When attaching a virtio disk to a VM, the iothread is missing.

To Reproduce

1. Configure IOTHREADS = "1" in the vmm_exec_kvm.conf.
2. Restart the opennebula service
3. Instantiate a VM
4. Attach a virtio disk to the VM. The disk is attached with:
   <driver name="qemu" type="raw" cache="none" io="native" discard="unmap" queues="4"/>

Expected behavior
The disk should have an iothread.

Details

• Affected Component: [Virtualization]
• Hypervisor: [KVM]
• Version: [7.0, 7.2]

Additional context
After restarting the VM via OpenNebula, there is an iothread defined in the disk definition:
<driver name="qemu" type="raw" cache="none" io="native" discard="unmap" iothread="1" queues="4"/>
I think "to have something rather than nothing" is better in this case. It is clear that replicating the round-robin logic across multiple places is hard, but a default value for a given dataplane inb the kvmrc file could be a good start.

Also, referring to my very, very old opinion that the right place for building the VM definition XML is in the VMM driver, not in the core of OpenNebula. Then the round-robin logic would be in one function used in both cases - on VM definition and on disk hotplug...

Progress Status

• Code committed
• Testing - QA
• Documentation (Release notes - resolved issues, compatibility, known issues)