imsm: load migration record from right disk

imsm: load migration record from right disk

Migration record is only stored on disks in first and second metadata
slot. The function to load the record incorrectly passes disk slot as
disk index. If rebuilt has taken place for a container, disk slot
doesn't match disk index so it causes migration record to be read from a
disk it has not been written to. As a result reshape operation fails.

Signed-off-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>