Bei Version 1.6 (zur Zeit im testing) hat sich das wohl geändert:
http://wiki.qemu.org/ChangeLog/1.6
Support for thin provisioning ("discard") on qcow2 images. When a device is opened with "-drive ...,discard=on", discard requests from the guest are forwarded to the underlying file system or block device as is the case for "raw" format images.