qemu: Reduce need to call qemuDomainGetSCSIControllerModel

qemu: Reduce need to call qemuDomainGetSCSIControllerModel

Now that post parse processing handles setting the SCSI controller
model, there's no need to call qemuDomainGetSCSIControllerModel to
get the "default controller" when building the command line controller
string or when assigning the spaprvio address since the controller
model value will already be filled in.