If you attempt to attach a CD-ROM drive to an available slot on a running virtual machine of domain type HVM, an error occurs and it is no longer possible to use the specified slot. For example, after you attempt to attach a CD-ROM drive to slot 2, you then attempt to attach a virtual disk to slot 2. In both cases an error occurs and slot 2 becomes unusable.
This issue occurs because running HVM virtual machines do not support the function to attach CD-ROM drives.
Workaround: Stop the virtual machine and then attach the CD-ROM drive.
Bug 21300944