libxl: use xc_topologyinfo to figure out how many CPUs we actually have

libxl: use xc_topologyinfo to figure out how many CPUs we actually have

Within libxl_get_cpu_topology(), considering all the CPUs the hypervisor
supports to be valid topology entries might lead to misleading and incorrect
behaviours, e.g., the output of `xl info -n' below on a 16 cores machine:
...
cpu_topology           :
cpu:    core    socket     node
  0:       0        1        0
  1:       0        1        0
  2:       1        1        0
  3:       1        1        0
  4:       9        1        0
  5:       9        1        0
  6:      10        1        0
  7:      10        1        0
  8:       0        0        1
  9:       0        0        1
 10:       1        0        1
 11:       1        0        1
 12:       9        0        1
 13:       9        0        1
 14:      10        0        1
 15:      10        0        1
 16:       0        0        0
 17:       0        0        0
 18:       0        0        0
 19:       0        0        0
 20:       0        0        0
 ...
 ...
 62:       0        0        0
 63:       0        0        0

However, xc_topologyinfo() tells us (in max_cpu_index) how many entries
arrays it returns corresponds to actually valid CPUs, so let's use that
information.

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