fdc: add disk field

Currently, 'drive' is used both to represent the current diskette
type as well as the current drive type.

This patch adds a 'disk' field that is updated explicitly to match
the type of the disk.

As of this patch, disk and drive are always the same, but forthcoming
patches to change the behavior of pick_geometry will invalidate this
assumption.

disk does not need to be migrated because it is not user-visible state
nor is it currently used for any calculations. It is purely informative,
and will be rebuilt automatically via fd_revalidate on the new host.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1453495865-9649-5-git-send-email-jsnow@redhat.com
