Use subject instead of invalid_subject because the validation assertions
test the subject double.
This also adds an additional check when validating the `size` attribute
because it is only required for disks of type `:disk`.
This builds on the existing disk functionality, and adds some special
IDE controller-related flavor.
Considerations for IDE controllers:
- Primary/secondary attachments, so that each port can have two devices
attached
- Adding the ability to address a specific controller name for disk
attachment
This also prevents a user from attaching multiple instances of the same
ISO file, because VirtualBox will assign each of these the same UUID
which makes disconnection difficult. However, if multiple copies of the
ISO are attached to different devices, removing the DVD config will
cause the duplicate devices to be removed.
We may want to consider additional work to make the storage controllers
truly generic.
Prior to this commit, the default value of disk_ext was set in the
finalize! method, and was really only valid for the virtualbox provider.
This commit updates that by moving the step into the validate function,
which has access to the machines provider.
