Disks should not become shared without explicit user consent

[shodanshok]

Currently, when an user add an already-in-use RAW volume to another virtual machine, the disk is attached as a "shared writable" volume and, at the same time, such a change is scheduled for the first virtual machine also. Sharing a disk in this manner poses a significant risks as, when the two virtual machines start, they will both try to open the underlying volume and corruption will quickly happen. The only warning the user see is a yellow/orange triangle and a (small) orange statement that the disk will become shared.

Changing a disk from "writeble" to "shared writable" should be done only after explicit user consent and never be easy to accidentally trigger. virt-manager does exactly that: it asks the user if the disk should really be added to another virtual machine and, after that, does not automatically marks the disk as shared. So user not only needs to confirm the add operation, but it has to enable the "shareable" flag on both machines before allowing them to start.

Please do the same for cockpit-machines. As a side note, the add disk via "custom path" form does not automatically change the "shareable" flag, so removing this behavior from the "use existing" disk form is going to present a more consistent (and much safer) behavior.