KVM: SVM: Add paranoid helper for checking if vCPU is AVIC-addressable

Add a helper to check if a vCPU is addressable by AVIC hardware, i.e. has
an APIC ID that fits in the physical ID table, and use the more paranoid
helper when determining if a vCPU is compatible with AVIC when initializing
the vCPU.  KVM is supposed to reject vCPU creation if the vCPU's ID is
greater than or equal to max_vcpu_ids, i.e. simply checking the
architectural maximum *should* suffice.  But piecing together why this is
safe is unnecessarily difficult, and there is no meaningful downside to
being extra cautious.

Suggested-by: Naveen N Rao (AMD) <naveen@kernel.org>
Reviewed-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://patch.msgid.link/20260923155108.1550622-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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