Use one function chosing spares from container

Use one function chosing spares from container

container_chose_spares in Monitor.c and
get_spares_for_grow in super-intel.c
do the same thing: search for spares in a container.

Another version will also be needed for Incremental
so a more general solution is presented here and
applied in two previous contexts.

Normally domlist==NULL would lead an empty list but
this is typically checked earlier so here it is interpreted
as "do not test domains".

Signed-off-by: Anna Czarnowska <anna.czarnowska@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>