KVM: s390: Simplify online vcpus counting for stsi

We don't need to loop over all cpus to get the number of
vcpus. Let's use the available counter online_vcpus instead.

Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>