speedup(image): Use dd to write image

- Converting the image to raw first takes around 2 seconds in a ramdisk,
  but using dd to image the disk speeds up imaging almost 2x by allowing
  use of direct IO and blocksizes.
- We may need, in the future, to be able to configure the options passed
  to dd based on the hardware we're imaging onto. These options have
  been confirmed as better than qemu-img on most hardware.
Merge pull request #48 from rackerlabs/jay/TwoStepImaging

speedup(image): Use dd to write image