KVM: nVMX: Dynamically compute max VMCS index for vmcs12

Calculate the max VMCS index for vmcs12 by walking the array to find the
actual max index.  Hardcoding the index is prone to bitrot, and the
calculation is only done on KVM bringup (albeit on every CPU, but there
aren't _that_ many null entries in the array).

Fixes: 3c0f9936

 ("KVM: nVMX: Add a TSC multiplier field in VMCS12")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210618214658.2700765-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>