Support user-configured bus type for VM

[arturshadnik]

Currently, forklift always assigns virtio as the bus type for disks when creating a KubeVirt VM. In some cases, it would be useful to have the ability to select a different bus.

virtio causes every disk to be assigned its own PCIe controller. This makes the disk itself show up with the exact same location information in Windows with regards to Port, Target and LUN values.
scsi will attach all disks to the same SCSI controller, and then set different LUN values for each disk
The latter (scsi) is a bit easier to see when just looking at the disk in Windows. The former (virtio) requires some Powershell trickery to figure out which disk is on which controller first
Also, virtio supports a max of 30 disks, while scsi supports orders or magnitude more.

It would be nice to have this configurable, in a similar way that boot disk currently is.
Valid bus types: virtio, scsi, sata