libvirt takes the same disk file or MAC twice in a persistent
definition. A double click or a resent request then attached an
existing volume twice, and a shut-off VM did not start again (QEMU
opens no file twice for writing), or added a second NIC with the MAC
the form had filled in. Attaching a volume that is already a disk of
the VM, live or in its next start, and adding a NIC with a MAC the VM
has are now refused with the reason.
