libxl: unset info->numa_placement upon successful placement

libxl: unset info->numa_placement upon successful placement

so that, if the same config is reused later, the following
two (good) things happen:
 - we do not trip over warnings because node and/or vcpu
   soft affinity now exist (as a consequence of the
   successful placement), but numa_placement is still
   true;
 - we end up always using the results of the original
   execution of the placement algorithm, rather than
   re-running it at each re-use of the same config,
   which is what most users expects and wants.

This fixes the bug reported here:
http://lists.xenproject.org/archives/html/xen-devel/2015-06/msg04454.html

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