x86/cpufeatures: Add CPUID feature bit for Idle HLT intercept

The Idle HLT Intercept feature allows for the HLT instruction
execution by a vCPU to be intercepted by the hypervisor only if there
are no pending events (V_INTR and V_NMI) for the vCPU. When the vCPU
is expected to service the pending events (V_INTR and V_NMI), the Idle
HLT intercept won’t trigger. The feature allows the hypervisor to
determine if the vCPU is idle and reduces wasteful VMEXITs.

In addition to the aforementioned use case, the Idle HLT intercept
feature is also used for enlightened guests who aim to securely manage
events without the hypervisor’s awareness. If a HLT occurs while
a virtual event is pending and the hypervisor is unaware of this
pending event (as could be the case with enlightened guests), the
absence of the Idle HLT intercept feature could result in a vCPU being
suspended indefinitely.

Presence of Idle HLT intercept feature for guests is indicated via CPUID
function 0x8000000A_EDX[30].

Signed-off-by: Manali Shukla <Manali.Shukla@amd.com>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250128124812.7324-2-manali.shukla@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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