KVM: TDX: Get system-wide info about TDX module on initialization

TDX KVM needs system-wide information about the TDX module. Generate the
data based on tdx_sysinfo td_conf CPUID data.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Co-developed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Signed-off-by: Tony Lindgren <tony.lindgren@linux.intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
---
 - Clarify comment about EAX[23:16] in td_init_cpuid_entry2() (Xiaoyao)
 - Add comment for configurable CPUID bits (Xiaoyao)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>