KVM: selftests: Track kvm_vcpu object in tsc_scaling_sync

Track the added 'struct kvm_vcpu' object in tsc_scaling_sync instead of
relying purely on the VM + vcpu_id combination.  Ideally, the test
wouldn't need to manually manage vCPUs, but the need to invoke a per-VM
ioctl before creating vCPUs is not handled by the selftests framework,
at least not yet...

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>