nova does not uncompress qcow compressed images

[Scott Moser]

Related bugs: bug 833265, bug 836759

if a compressed qcow image is uploaded, then nova does not decompress it before using it either as a backing image (if FLAGS.use_cow_images) or as the disk image.

The user may want to use compressed disk images for transmission, but it is unlikely that they want to incur the overhead cost of all reads to that disk going through a cpu decompression.

For Ubuntu, we want to distribute disk images that are sparse and compressed, and that are as close to immediately useful as possible.

I suggest that if an image is a compressed disk, it should be decompressed before being used as a backing device.