imsm: use same slot across container

imsm: use same slot across container

Autolayout relies on drives order on super->disks list, but
it is not quaranted by readdir() in sysfs_read(). As a result
drive could be put in different slot in second volume.

Make it consistent by reffering to first volume, if exists.

Use enum imsm_status to unify error handling.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>