xen: allow for explicitly specifying node-affinity

xen: allow for explicitly specifying node-affinity

Make it possible to pass the node-affinity of a domain to the hypervisor
from the upper layers, instead of always being computed automatically.

Note that this also required generalizing the Flask hooks for setting
and getting the affinity, so that they now deal with both vcpu and
node affinity.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Juergen Gross <juergen.gross@ts.fujitsu.com>
Acked-by: Keir Fraser <keir@xen.org>