KVM: TDX: Handle TDX PV CPUID hypercall

KVM: TDX: Handle TDX PV CPUID hypercall

Handle TDX PV CPUID hypercall for the CPUIDs virtualized by VMM
according to TDX Guest Host Communication Interface (GHCI).

For TDX, most CPUID leaf/sub-leaf combinations are virtualized by
the TDX module while some trigger #VE.  On #VE, TDX guest can issue
TDG.VP.VMCALL<INSTRUCTION.CPUID> (same value as EXIT_REASON_CPUID)
to request VMM to emulate CPUID operation.

Morph PV CPUID hypercall to EXIT_REASON_CPUID and wire up  to the KVM
backend function.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
[binbin: rewrite changelog]
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Message-ID: <20250227012021.1778144-6-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>