imsm: spare devices are represented as single disk containers

This poses a small problem for the case of handling multiple raid1 arrays
across separate disk pairs i.e. 2 mirrors on 4 disks.  The option-ROM will
configure this as two containers.  We may need the capability for one
container to ask for an unused spare in another container.  For now spares
will just maintain the affinity established at assemble time.

To support this configuration spare devices must be allowed to be assembled
into the container even though the metadata indicates the disk belongs to a
different family.

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