Cope with missing /sys/devices/system/cpu/cpu0/topology files

Cope with missing /sys/devices/system/cpu/cpu0/topology files

Not all kernel builds have any entries under the location
/sys/devices/system/cpu/cpu0/topology. We already cope with
that being missing in some cases, but not all. Update the
code which looks for thread_siblings to cope with the missing
file

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>