KVM: selftests: Convert hardware_disable_test to pass around vCPU objects

Pass around 'struct kvm_vcpu' objects in hardware_disable_test instead of
the VM+vcpu_id (called "index" by the test).

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