oci-poc-vms init: fix second drive index

If qemu starts with a second drive index cannot be equal to 0, since that would be already used by the first drive and qemu would fail like this:

qemu-system-x86_64: drive with bus=0, unit=0 (index=0) exists

thus no slave vms will be spawned.

This commit changes index for second drives used by slave vms.