VM "last started" date

[andrewdavidwong]

Similar to the "last backup" column in Qubes Manager, allow the user to see when a VM was last started (if ever). This would be especially useful when trying to decide whether a VM needs to be included in a backup. (If the "last started" date is the same or earlier than the "last backup" date, then it probably doesn't need to be backed up again.)