KVM: selftests: Use vm_create() in tsc_scaling_sync

Use vm_create() instead of vm_create_default_with_vcpus() in
tsc_scaling_sync.  The existing call doesn't create any vCPUs, and the
guest_code() entry point is set when vm_vcpu_add_default() is invoked.

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