supporting virtio-scsi image disk

[cxfcxf]

in some cause, people want to use sd? as dev name

<devices>
 <disk type='file' device='disk'>
  <target dev='sda' bus='scsi'/>
  <address type='drive' controller='0' bus='0' target='0' unit='0'/>
 </disk>
 <controller type='scsi' index='0' model='virtio-scsi'/>
</devices>

and
say if you create a raw image, and want to use it on domain, it will failure due to default domain xml only support qcow2,

1 error(s) occurred:

* libvirt_domain.fopd-tst1088-lab1: 1 error(s) occurred:

* libvirt_domain.fopd-tst1088-lab1: Error creating libvirt domain: virError(Code=1, Domain=10, Message='internal error: qemu unexpectedly closed the monitor: 2018-03-09T17:35:26.716475Z qemu-kvm: -drive file=/data/raid/sda/images/tst00.lab1-0,format=qcow2,if=none,id=drive-scsi0-0-0-0: Image is not in qcow2 format')

with config support like this

  disk {
       volume_id = "${libvirt_volume.fopd-tst0023-lab1-0.id}",
       driver_type = "raw",
       scsi = true
  }