WIP azure: finalize azure image

The VHDs uploaded into azure must be sized a multiple of 1MB and the kernel
needs some custom boot parameters.

I still need to figure out dynamically sized images. Otherwise, we'll be pushing 8GiB over the network...
Also related:

```python
  # Sometimes qemu-img expects disks sizes aligned to 64k
  align_bytes = metadata['alignment'] * metadata['block_size']
  if align_bytes < 65536 or align_bytes % 65536 != 0:
    raise InvalidLayout('Invalid alignment, 64KB or better required')
```