[Feature request] qemu-img option about recompressing

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/603878

Suppose I have a fresh compressed qcow2 image. After some time the data
were recorded without compression. I decide to make "QEMU-IMG convert" for
that image to reduce its size.

I want a new option, which selects between the two algorithms overdriven
images:
1. extract all / compress again when converting images (in the current
implementation)
2. compress only uncompressed blocks and just copy the compressed blocks
without re-compression.

This option is only needed when converting compressed image to compressed
and the compression algorithm is the same.