Description of problem:
Failed to hotplug volume to VM with IOThreads

Version-Release number of selected component (if applicable):
CNV 4.8.1

How reproducible:
100%

Steps to Reproduce:
1. Create PV and VM, details in attached yaml files.
# oc create -f asb-pv-dv-nfs-rhel.yaml
persistentvolume/asb-pv-dv-nfs-rhel created

# oc create -f asb-vm-dv-nfs.yaml
virtualmachine.kubevirt.io/asb-vm-dv-nfs-rhel created

# oc get vmi
NAME                 AGE     PHASE     IP             NODENAME
asb-vm-dv-nfs-rhel   3m59s   Running   10.128.0.137   dell-per730-64.lab.eng.pek2.redhat.com

2. Login to VM console, there is 1 disk /dev/vda, check the VM xml,
it with iothread settings.

# virtctl console asb-vm-dv-nfs-rhel
check there is disk /dev/vda

# oc rsh virt-launcher-asb-vm-dv-nfs-rhel-c2qdn
sh-4.4# virsh dumpxml 1|grep disk -A 8
    <disk type='file' device='disk' model='virtio-non-transitional'>
      <driver name='qemu' type='raw' cache='none' error_policy='stop' discard='unmap' iothread='1' queues='2'/>
      <source file='/var/run/kubevirt-private/vmi-disks/rootdisk/disk.img' index='1'/>
      <backingStore/>
      <target dev='vda' bus='virtio'/>
      <boot order='1'/>
      <alias name='ua-rootdisk'/>
      <address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
    </disk>


3. Create PV and PVC, details in attached yaml files.
# oc create -f asb-pv-disk1.yaml
# oc create -f asb-pvc-disk1.yaml

# oc get pv
NAME                 CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS   CLAIM                           STORAGECLASS   REASON   AGE
asb-pv-disk1         1Gi        RWX            Retain           Bound    openshift-cnv/asb-pvc-disk1     nfs                     122m
asb-pv-dv-nfs-rhel   12Gi       RWX            Retain           Bound    openshift-cnv/asb-dv-nfs-rhel   nfs                     7m14s

# oc get pvc
NAME              STATUS   VOLUME               CAPACITY   ACCESS MODES   STORAGECLASS   AGE
asb-dv-nfs-rhel   Bound    asb-pv-dv-nfs-rhel   12Gi       RWX            nfs            7m8s
asb-pvc-disk1     Bound    asb-pv-disk1         1Gi        RWX            nfs            122m

4. Try to hotplug volume to VM by command line without error.
# virtctl addvolume asb-vm-dv-nfs-rhel --volume-name=asb-pvc-disk1 --serial=123456

5. Check hp-volume pod is running, oc describe vmi, there is libvirt error:
"virt-handler               server error. command SyncVMI failed: "LibvirtError(Code=67, Domain=10, Message='unsupported configuration: IOThreads not available for bus scsi target sda')""

# oc describe vmi >vmi.yaml
------------------------------------------------------
  Warning  SyncFailed        4m26s (x15 over 4m41s)  virt-handler               server error. command SyncVMI failed: "LibvirtError(Code=67, Domain=10, Message='unsupported configuration: IOThreads not available for bus scsi target sda')"
------------------------------------------------------

# oc get pod|grep volume
hp-volume-fbb5r                                       1/1     Running   0          39s

# oc get vmi asb-vm-dv-nfs-rhel -o yaml >asb-vm-dv-nfs-rhel-vmi.yaml
-----------------------------------
  volumeStatus:
  - hotplugVolume:
      attachPodName: hp-volume-fbb5r
      attachPodUID: bfffd77c-f8c6-44ef-bc23-cecac3d8bb88
    message: Volume asb-pvc-disk1 has been mounted in virt-launcher pod
    name: asb-pvc-disk1
    phase: MountedToPod
    reason: VolumeMountedToPod
    target: ""
  - name: rootdisk
    target: vda
---------------------------------

6. Login to VM, there is no new attached disk, 
check the guest xml, there is no new attached disk.

7. Remove the "dedicatedIOThread: true" in asb-vm-dv-nfs.yaml, redo step1-5, no libvirt error, login to VM, there is new attached disk /dev/sda,
check the guest xml, there is new attached disk.

-------------------------------------------------------------
Disk /dev/sda: 967 MiB, 1013972992 bytes, 1980416 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

    volumeStatus:
    - hotplugVolume:
        attachPodName: hp-volume-w6tf8
        attachPodUID: fa924df7-d77d-4862-aa23-58ebe49608df
      message: Successfully attach hotplugged volume asb-pvc-disk1 to VM
      name: asb-pvc-disk1
      phase: Ready
      reason: VolumeReady
      target: sda
    - name: rootdisk
      target: vda

    <disk type='file' device='disk' model='virtio-non-transitional'>
      <driver name='qemu' type='raw' cache='none' error_policy='stop' discard='unmap' queues='2'/>
      <source file='/var/run/kubevirt-private/vmi-disks/rootdisk/disk.img' index='1'/>
      <backingStore/>
      <target dev='vda' bus='virtio'/>
      <boot order='1'/>
      <alias name='ua-rootdisk'/>
      <address type='pci' domain='0x0000' bus='0x04' slot='0x00' function='0x0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' error_policy='stop' discard='unmap'/>
      <source file='/var/run/kubevirt/hotplug-disks/asb-pvc-disk1/disk.img' index='2'/>
      <backingStore/>
      <target dev='sda' bus='scsi'/>
      <serial>123456</serial>
      <alias name='ua-asb-pvc-disk1'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

Actual results:
1. Hotplug volume by command line is using bus=scsi by default
2. In step 6, hit libvirt error, there is no new attached disk, check the VM xml, there is no new attached disk

Expected results:
1. Hotplug volume by command line use bus=virtio by default or let user to set the bus value
2. In step 6, there is new attached disk in VM and VM xml.

Additional info:
- yaml files in step1-3 are attached to this bug