Set SCSI as the default cdrom bus for PowerKVM

Set the default cdrom bus to scsi when the guest architecture is
ppc64 and the hypervisor is kvm|qemu. Checks the arch from
image_meta. Fallback to host arch if arch is not present in the
metadata associated with the image.

KVM unit tests without image_meta unified in a loop.

Change-Id: I62d1490ee8c9ba45a087f934342f14386ddb7cb2
Merge "Set SCSI as the default cdrom bus for PowerKVM"