No warning when adding the same disk image to multiple virtual machines

[shodanshok]

When adding the same disk image to two or more virtual machines, virt-manager alerts the user with the following message:

Disk <diskname> is already in use by other guests [<guestlist>]

This message can be very useful to avoid overwriting a disk from a powered off virtual machine, so it would be great to have cockpit-machines emit the same warning.