This feature would enbale endusers to request a given cinder volume use a diffent disk bus based on the performance and feature needs.

for example using virtio-blk by default but using virtio-scsi for ceph backed volumes.
in the future this could enable vdpa offload on a per volume basis by supporting hw_disk_bus=vdpa

as per the ptg disucssion
https://etherpad.opendev.org/p/r.22d95e20f7133350dcad573c015ed7da#L180

nova should use the cinder volume metadata to read the prefered disk bus.
nova can in the future expand this to other relevent image propertiese.
i.e. enabling virtio blk multi queue on a per device basis once that feature is supported in nova.

this should follow the same precenece rules as we do for per port numa affintiy

if hw_disk_bus is set in the volume metadata we should use that
if not we should fall back to the existing image porperty
if not we fallback to the libos-info default and finally
we fall back to the libvirt driver default (virtio-blk) if no model is specified.