KVM: x86: Expose Zhaoxin RNG2 CPUID feature

Advertise the Zhaoxin second-generation hardware RNG to guests via
CPUID 0xC0000001 EDX bits 22 (RNG2) and 23 (RNG2_EN).

RNG2 is exposed by the REP XRNG2 instruction (encoding F3 0F A7 F8),
documented in the Zhaoxin PadLock Instruction Reference, subsection 1.3
("REP XRNG2"). It produces random bytes from two on-die RNG sources
selectable via RAX bits[10:9] and an output mode (raw vs post-processed)
controlled by RDX bits[1:0], providing high-quality entropy intended
for cryptographic operations.

REP XRNG2 is unprivileged (no CPL restriction) and available in all
CPU modes, with no associated MSR control. The RNG2 and RNG2_EN bits
are redundant by hardware design (set or cleared together) and both
serve purely as CPUID-level feature-presence reporting flags requiring
no KVM emulation. Both bits are advertised because different software
may probe either one when checking for RNG2 availability.

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Ewan Hai <ewanhai-oc@zhaoxin.com>
Link: https://patch.msgid.link/20260610023512.3690734-4-ewanhai-oc@zhaoxin.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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