libxl: automatic placement deals with node-affinity

libxl: automatic placement deals with node-affinity

Which basically means the following two things:
 1) during domain creation, it is the node-affinity of
    the domain --rather than the vcpu-affinities of its
    VCPUs-- that is affected by automatic placement;
 2) during automatic placement, when counting how many
    VCPUs are already "bound" to a placement candidate
    (as part of the process of choosing the best
    candidate), both vcpu-affinity and node-affinity
    are considered.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Juergen Gross <juergen.gross@ts.fujitsu.com>