KVM: TDX: Make TDX VM type supported

Now all the necessary code for TDX is in place, it's ready to run TDX
guest.  Advertise the VM type of KVM_X86_TDX_VM so that the user space
VMM like QEMU can start to use it.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
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TDX "the rest" v2:
- No change.

TDX "the rest" v1:
- Move down to the end of patch series.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>