kvm_stat: Only consider online cpus

kvm_stat: Only consider online cpus

In kvm_stat we grovel through /sys to find out how many cpus are in the
system. However if a cpu is offline it will still be present in /sys,
and the perf_event_open() will fail.

Modify the logic to only return online cpus. We need to be careful on
systems which don't support cpu hotplug, the online file will not be
present at all.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>