Place explicitly-declared block device drivers at the end of the PCI bus

In 1745eff4 we risk reorganizing the
list of PCI devices, due to how raw -drive disks end up placing their
parent devices at the end of the PCI bus.

Instead, we should explicitly place the directives to configure the
block device interfaces at the tail of the PCI bus.

This should hopefully address the final concerns in
dkg/kvm-manager#7