platform/machine/qemu/cluster: create machine with multiple devices
platform/machine/qemu/cluster: create machine with multiple devices

Adds the ability to create a machine with multiple disk drives
attached. Default functionality is unchanged as the original
`NewMachine` function has been renamed to `NewMachineWithOptions`
which takes an additional argument of type `MachineOptions`. A
wrapper function now exists at `NewMachine` which calls
`NewMachineWithOptions` with an empty `MachineOptions` struct.

This only implements an `AdditionalDisks` option on the
`MachineOptions` struct, which itself only implements a struct
containing a `Size` string. The `Size` string represents the disk
image size in bytes, with optional suffixes "K" (kilobyte, 1024),
"M" (megabyte, 1024K), "G" (gigabyte, 1024M), and "T" (terabyte,
1024G).

Whenever additional disks are present they will have a new `qcow2`
image created for them which is immediately cleaned up similar to
how we are doing for the primary disk currently.

dm0-/mantle,glevand/coreos--mantle,dm0-/mantle,glevand/coreos--mantle