I have bad news, this is 100% reproductible (at leat on 2 platforms for the moment, will have a third soon to be upgraded to 5.4). This happens with Ssh AND qcow2 type datastores.
Did you try to reproduce ?
Create 1 vm (we are using centos 7 kvm template from marketplace)
While the VM is running, go to storage tab, click on attach disk
Choose volatile, type 1 GB for the size, and just change Filesystem format to qcow2
Click on attach
The page will refresh to show the VM is now in HOTPLUG state, and you can see the new disk is added but in raw format.
After a few second, if you refresh, state is back to RUNNING, adding the disk has failed tho. Errors in log say image is not in qcow2 format.
We are using CentOS7
Should I open a bug report ?