qemu-img: support ZSTD compression level customization

Goal

Hi, when qemu-img convert -c -o compression_type=zstd is called, there is no parameter to customize the compression level, the default (level 3) is used. It would be nice to be able to change the level via a command-line option. In some scenarios where one image is used by a lot of systems, having the best possible compression level could really greatly reduce bandwidth usage.

Technical details

I believe the implementation should be done in qcow2_zstd_compress: https://gitlab.com/qemu-project/qemu/-/blob/v8.1.1/block/qcow2-threads.c#L188