Add options for --disk-only, --quiesce and --memspec to snapshot UI

[shodanshok]

--disk-only, --quiesce and --memspec are important options for snapshots done via virsh. Now that both virsh and cockpit-machines fully supports external snapshots, please provide the relative options/checkboxes in the snapshot creation UI.

--disk-only is especially valuable as, not dumping memory, it completes far faster and the VM experiences no downtime at all. On the other hand, memory-dumping a VM with large RAM will bring it offline for multiple seconds (or even minutes).

--quiesce can be very useful to get crash-consistent snapshots without the overhead of saving the entire VM RAM content.

--memspec can be hidden behind a (default-selected) "internal" checkbox for the normal case, as the to-be-created qcow2 files can store the memory dump inside itself - avoiding polluting the storage directory with separate memory dump files and simplifying snapshot files management. For advanced cases, users can uncheck the "internal" checkbox and specify the full memory dump path (as now).

Thanks.