When adding the new LXC config format we forgot to adapt the
get_start_stop_list and do s/pve.startup/onboot/

This fixes that qemu VMs were started always before LXC CTs,
irregardless of the defined order.

This also let's us simplify the surrounding code and reduce code
reuse.
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>