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
1 file changed