Improve media disk question default

Improve media disk question default

Sets on an unmounted disk partition are most likely to be on install media,
e.g. CD or USB stick.

Default 'Which disk contains the install media?' to the first disk that is
a) not the root disk and b) not a disk with softraid chunks (hosting the
root disk, for example).

This makes it point at the right disk in most setups.
All disks remain valid answers, this is only about the default.

OK afresh1