Change disk bus from scsi to virtio to gain performance

Verify the kernel module is loaded with the VM image
$ lsmod | grep virtio_blk

Signed-off-by: JenTing Hsiao <[email protected]>

Author: jenting

Diff for: .werft/vm/manifests.ts

@@ -73,7 +73,7 @@ spec:
             - name: system
               bootOrder: 1
               disk:
-                bus: scsi
+                bus: virtio
             - name: cloudinitdisk
               disk:
                 bus: virtio