[Qemu-block] RFC chaning zlib windowBits for QCOW2

[Peter Lieven]

Hi,

I was experimenting a little with optimized zlib versions (like zlib-ng) to 
optimize the speed of compressed qcow2 generation.

I found out that chaning the windowBits from -12 to -15 alone increases 
compression speed by about 15% while lowering
the compressed size by about 6%. As a test case I used a Debian 8.1 disk image.

What is your opinion in chaning the windowBits? The cost is a higher memory usage. 
The usage changes from 272k -> 384k for
compression and from 4k -> 32k for decompression.

Peter