This feature worked fine in previous versions of CentOS (6.0-6.4), however starting with 6.5 qemu-img no longer recognises the "-s" parameter that Cloudstack uses to take the volume snapshots.
This problem can be worked around in many ways, for example by downgrading qemu-img to the 6.4 version, but this idea may not appeal to those who like to stay up to date.
Another more elegant workaround that I've discovered since getting my hands dirty with ACS is that the script which is responsible for taking the snapshot first looks for a "cloud-qemu-img" in the $PATH, if it can't find any it will fallback on whatever `which qemu-img` returns. So, the solution is as simple as getting the old qemu-img installed as cloud-qemu-img; this can be done like this:
mkdir cloud-qemu-img cd cloud-qemu-img wget http://vault.centos.org/6.4/updates/x86_64/Packages/qemu-img-0.12.1.2-2.355.el6_4_4.1.x86_64.rpm rpm2cpio qemu-img-0.12.1.2-2.355.el6_4_4.1.x86_64.rpm |cpio -idmv cp ./usr/bin/qemu-img /usr/bin/cloud-qemu-imgVoilà! This is probably the best solution because it doesn't modify the Cloudstack script nor does it interfere with the stock qemu packages.
 - /usr/share/cloudstack-common/scripts/storage/qcow2/managesnapshot.sh