Set default disk driver to virtio-blk on Power

Reason is to be identical to the upstream KVM CI. Some Tempest
tests rely on vdX virtio-blk device naming. Others simply create
their own with a brand new image. Also, the scsi support on the
CirrOS image is limited, tests booting from volume fail.

Change-Id: I389147a58042aa6098a695e6dd32f3e697fbbbab
diff --git a/functions b/functions
index 8b69c73..20b83b3 100644
--- a/functions
+++ b/functions
@@ -364,7 +364,7 @@
     esac
 
     if is_arch "ppc64le" || is_arch "ppc64" || is_arch "ppc"; then
-        img_property="--property hw_disk_bus=scsi --property hw_scsi_model=virtio-scsi --property hw_cdrom_bus=scsi --property os_command_line=console=hvc0"
+        img_property="--property hw_cdrom_bus=scsi --property os_command_line=console=hvc0"
     fi
 
     if is_arch "aarch64"; then