Set correct default disk bus back to virtio on ppc64

virtio is supported and should be the default disk bus on Power to
take advantage of I/O performance drivers. This aligns with Nova
default bus values on PowerKVM. SCSI is the default for cdrom.

Change-Id: I5de08c90359b3a500c352c09c07b6b082ddb4325
Merge "Set correct default disk bus back to virtio on ppc64"