Treat all devices at the container level as spares

Treat all devices at the container level as spares

Raid disk and disk number information is not relevant at the container
level, especially for imsm.  So arrange for getinfo_super_imsm() to
always publish devices as spares and report the number of spares at
Assemble() time.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>