core, engine: Fix CD-ROM payload in ppc64 VMs

This change fixes the addressing for payload CD-ROMs in ppc64 VMs,
previously there was not a way to address two virtual CD drives at the
same time in the sPAPR VSCSI controller.

Change-Id: Ieca8008621392b95cb9dcd5eb8ca230233de19dc
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1073540
Signed-off-by: Vitor de Lima <vitor.lima@eldorado.org.br>