add support for a disks attrib, providing ability to configure an array of disks

[mco-gh]

Eric writes: maybe disk should be a published property that is an object? Polymer [de]serializes objects:

reply: I like the current attribs for the boot disk because it provides easier usability but I'd also like to add a disk object for power users. Beyond that, one can start a VM with auxiliary disks beyond the boot drive, and there's no way to express that with my current attribute set. Having the disk attribute be a "disks" (plural) attrib, which takes an array of json objects would be very powerful. I'll add an issue to do that but don't want to try to sweep that in now as I'm crunching this for next week's codelab.