Support zvols as raw block volumes

[candlerb]

k8s now has raw block volume support, which would map nicely to zvols. It would be useful for kubevirt amongst others.

The volume provisioning doc explicitly says this can be done via external provisioner:

Kubernetes v1.9 or later have functionality to deploy raw block volume instead of filesystem volume as a new feature. To support the feature, we added volumeMode parameter which takes values Filesystem and Block to pv.Spec and pvc.Spec. In order to deploy block volume via external provisioner, following conditions are REQUIRED.

• A storage has ability to create raw block type of volume
• Block volume feature has been supported by the volume plugin
• External-provisioner MUST set volumeMode which matches requirements in claim.Spec into pv.Spec.