libvirt: add virtio-scsi disk interface support

A new virtio-scsi cotroller will be presented to the guests,
when hw_disk_bus_model will be set to virtio-scsi in Glance
image metadata:

glance image-update \
       --property hw_scsi_model=virtio-scsi \
       [image]

DocImpact
Partially implements blueprint libvirt-virtio-scsi-driver
Change-Id: I2723494130055687a9dfc58b1a27e586116fdff3
Merge "libvirt: add virtio-scsi disk interface support"