disk_util: ensure partitions are layed out on disk in order

So far the default iteration order of python dicts has mostly matched
the order that we want the partitions on disk but this is not always the
case. I caught the BIOS-BOOT partition being ordered on disk after the
USR-A partition. Nothing bad came of this but consistancy is good.
disk_layout: align partitions and disk sizes to 2MB

The VHD format actually uses 2MB blocks internally so the 1MB alignment
used in e77e4e54 wasn't sufficent to prevent other tools from further
adjusting the image size to align it. Additionally a 1MB alignment may
be triggering a bug in OpenStack or XenServer disk resizing that renders
that partial block at the end of the old image size unmapped/unavailabe.
Merge pull request #347 from marineam/align

Update disk/partition alignment, hopefully resolves issues with VHD images