KVM: x86: Virtualize AMD CPUID faulting

KVM: x86: Virtualize AMD CPUID faulting

On AMD CPUs, CPUID faulting support is advertised via
CPUID.80000021H:EAX.CpuidUserDis[bit 17] and enabled by setting
HWCR.CpuidUserDis[bit 35].

Advertise the feature to userspace regardless of host CPU support. Allow
writes to HWCR to set bit 35 when the guest CPUID advertises
CpuidUserDis. Update cpuid_fault_enabled() to check HWCR.CpuidUserDis as
well as MSR_FEATURE_ENABLES.CPUID_GP_ON_CPL_GT_0.

Unlike VMX, SVM prioritizes the CPUID intercept over the #GP induced by
CPUID faulting.[1] This behavior has been confirmed on a Turin CPU (F/M/S
1AH/2/1).

Link: https://lore.kernel.org/r/DS7PR12MB82011943131DF5415365E19E940B2@DS7PR12MB8201.namprd12.prod.outlook.com
Signed-off-by: Jim Mattson <jmattson@google.com>
Link: https://patch.msgid.link/20260527174347.2356165-5-jmattson@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>