Error message for transient commit can be confusing

[ALSchwalm]

When using the same location as the vmstore and the image-backend, an attempt to commit a VM as an image name that is the same as an existing VM will result in the following error:

An image with the name 'example' already exists

But this isn't really true. In fact there is a VM with the name 'example' and the vmstore is the same as the image-backend, causing the conflict. We should make these messages more clear, and probably start prefixing the VMs and images so they can't conflict