derive NUMA node affinity from hard and soft CPU affinity

derive NUMA node affinity from hard and soft CPU affinity

if a domain's NUMA node-affinity (which is what controls
memory allocations) is provided by the user/toolstack, it
just is not touched. However, if the user does not say
anything, leaving it all to Xen, let's compute it in the
following way:

 1. cpupool's cpus & hard-affinity & soft-affinity
 2. if (1) is empty: cpupool's cpus & hard-affinity

This guarantees memory to be allocated from the narrowest
possible set of NUMA nodes, ad makes it relatively easy to
set up NUMA-aware scheduling on top of soft affinity.

Note that such 'narrowest set' is guaranteed to be non-empty.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>