KVM: TDX: Handle TDX PV rdmsr/wrmsr hypercall

KVM: TDX: Handle TDX PV rdmsr/wrmsr hypercall

Morph PV RDMSR/WRMSR hypercall to EXIT_REASON_MSR_{READ,WRITE} and
wire up KVM backend functions.

For complete_emulated_msr() callback, instead of injecting #GP on error,
implement tdx_complete_emulated_msr() to set return code on error.  Also
set return value on MSR read according to the values from kvm x86
registers.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250227012021.1778144-10-binbin.wu@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>