Allowing inclusion of writethough disks in snapshotted machines

[Terry Ellison]

Currently VBox throws an error "Cannot take a snapshot when there is a Writethrough hard disk attached" if you attempt to snapshot a machine which includes a write-through hdd. This is different to the approach taken by VMware. There are scenarios where it makes a lot of sense to allow write-through hdds to be attached to a machine that you want to snapshot and rollback, for example if you are testing but want to log test outputs permanently and which you therefore want to store on a write-through hdd.

VBox should permit you to snapshot a VM which has a write-through hdd attached, but raise a warning that "snapshot will exclude attached writethrough hard disk. Changes to writethrough hard disk will be permanent".

[Frank Mehnert]

Duplicate of #597.