Why use vmi instead of vm for hotplug volume？

[haoxiaoci]

Is your feature request related to a problem? Please describe: when a virtualmachineinstance has been restart, because of hotplug volume is on vmi, hotplug volume disappeared, and this cause pod mount pvc failed in tenant cluster. detail: https://github.com/kubevirt/csi-driver/issues/110

Describe the solution you'd like:

1. implement csi-driver ListVolumes()， use external-attacher period resync to check if a volume is on a vmi, if volume actual status didn't match the volumeattachment status，external-attacher will be force resync the volume which will call ControllerPublishVolume(), hotplug volume again.
2. use virtualmachine instead of using virtualmachineinstance. i found issue that use vmi instead of vm(https://github.com/kubevirt/csi-driver/pull/23), Are there any concerns about not using vm?

Describe alternatives you've considered: A clear and concise description of any alternative solutions or features you've considered.

Additional context: Add any other context or screenshots about the feature request here.