Setup VM's boot order.

In order to prevent the VM to boot *only* by virtual disk during the
first boot, setup the boot order to: 1st option: disk; 2nd option: cdrom.

This configuration guarantees that the virtual firmware will boot by
cdrom after check there is no boot section on default disk.

Signed-off-by: Paulo Vital <pvital@linux.vnet.ibm.com>