Monitor: Include containers in spare migration

Monitor: Include containers in spare migration

Spare migration doesn't work for external metadata. mdadm skips
a container with spare device because it is inactive. It used to work
because GET_ARRAY_INFO ioctl returned valid structure for a container
and mdadm treated such response as active container. Current
implementation checks it in sysfs where container is shown as inactive.

Adapt sysfs implementation to work the same way as ioctl.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>