Write image directly to disk

Write image directly to disk by telling qemu-img that the output
file is the disk. Still uses O_DIRECT directly within qemu-img.

This avoids the need to have a huge amount of system memory to be
able to deploy an image that contains a huge disk.

The only feature left behind by that commit is the 64K block size.

Change-Id: If60bcd82581b2ea76c4acbc3eaa3f317d2ee6590