xend: fix automatic vcpu NUMA placement

xend: fix automatic vcpu NUMA placement

The automatic NUMA placement code (find_relaxed_node) in
XendDomainInfo.py contains several bugs:
1. The domain just created shall not be considered in the load
calculation.
2. Nodes with too less memory for the domain must be considered during
the load calculation phase and abandoned later, currently these nodes
win with a load of 0.
3. CPUs which are offline shall not be considered in load calculation.
4. (Integer-)Dividing the load value by the number of cores per node
may conceal subtle load differences resulting in a non-optimal node to
be chosen.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>