selftests: kvm: Simplify loop in kvm_create_max_vcpus test

On kvm_create_max_vcpus test remove unneeded local
variable in the loop that add vcpus to the VM.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>