Support qcow with qcow-rs

The [qcow-rs crate](https://github.com/panda-re/qcow-rs) offers read-only access for qcow images. I guess this might rely on downloading the entire qcow image (or at least, downloading its pieces sequentially?), but it seems like it could be a nice feature to add to SyncFileBuffer or AsyncXHRBuffer, or an alternative to those (SyncQCOW, AsyncXHRQcow). It looks like QCOW files are divided into clusters which are compressed independently of each other, so the code wouldn't necessarily be that different from the partfile or async buffer implementations. I believe cluster sizes are between 512 bytes and 2 MB.

Is this worth investigating further? Maybe waiting on a library that implements both reading and writing of qcow images before pursuing this would make sense, but I do think that the read feature on its own could be useful for bandwidth reasons since qcow images elide empty space.