fdc: add pick_drive

fdc: add pick_drive

Split apart pick_geometry by creating a pick_drive routine that will only
ever called during device bring-up instead of relying on pick_geometry to
be used in both cases.

With this change, the drive field is changed to be 'write once'. It is
not altered after the initialization routines exit.

media_validated does not need to be migrated. The target VM
will just revalidate the media on post_load anyway.

Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: John Snow <[email protected]>
Message-id: 1453495865[email protected]