Allow attached device to be converted from persistent to non-persistent
and the other way around.
This is to allow starting a VM with some device attached temporarily.
When VM is not running, it is possible to attach device only
persistently, so this change will allow to do that, then, after starting
the VM, change it to non-persistent - so it will not be attached again
at further startups.
QubesOS/qubes-issues#3055