xen: turn numa=on by default

xen: turn numa=on by default

I did some benchmark runs (lmbench & kernel compile) with a number of
guests running in parallel to compare the performance of numa=on vs.
numa=off.  As soon as one starts to load the machine, the performance
goes down in the numa=off case.  The tests were done on an 8-node
machine (4 cores each).  lmbench (actually copying large amounts of
memory) shows a dramatic dropdown, but I even noticed significant
performance decrease for a tmpfs based Linux kernel compile. Here a
summary of the data:

lmbench's rd benchmark (normalized to native Linux (=100)):
guests    numa=off       numa=on       avg increase
        min  avg  max  min  avg  max
     1       78.0           102.3
     7  37.4 45.6 62.0 90.6 102.3 110.9 124.4%
    15  21.0 25.8 31.7 41.7 48.7 54.1    88.2%
    23  13.4 17.5 23.2 25.0 28.0 30.1    60.2%

kernel compile in tmpfs, 1 VCPU, 2GB RAM, average of elapsed time:
guests    numa=off   numa=on   increase
      1    480.610    464.320    3.4%
      7    482.109    461.721    4.2%
     15    515.297    477.669    7.3%
     23    548.427    495.180    9.7%
again with 2 VCPUs and make -j2:
      1    264.580    261.690    1.1%
      7    279.763    258.907    7.7%
     15    330.385    272.762   17.4%
     23    463.510    390.547   15.7% (46 VCPUs on 32pCPUs)

Selected tests on a 4-node machine showed similar behavior (7.9 %
increase with 6 parallel guests on the 2 VCPU kernel compile
benchmark).

Note that this does not affect non-NUMA machines at all, since NUMA
will be turned off again by the code if no NUMA topology is detected.

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