libxl: have NUMA placement deal with cpupools

libxl: have NUMA placement deal with cpupools

In such a way that only the cpus belonging to the cpupool of the
domain being placed are considered for the placement itself.

This happens by filtering out all the nodes in which the cpupool
has not any cpu from the placement candidates. After that ---as
cpu pooling not necessarily happens at NUMA nodes boundaries--- we
also make sure only the actual cpus that are part of the pool are
considered when counting how much processors a placement candidate
provides.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>