Build images using loopdev instead of qemu-nbd.

Qemu-nbd does not perform well with older versions of qemu due to
the lack of writeback caching mode. It also only builds qcow2 images
and there is a desire for raw image support. Finally, qemu-nbd makes
it very difficult to build images concurrently due to the somewhat
opaque nature of how it selects a /dev/nbd# device. losetup, on
the other hand, makes this process very straight forward.

Change-Id: I309fad8af4fd1e8d1720c17b65e1897a76d5e897
Co-Author: Clint Byrum <clint@fewbar.com>