KVM: SVM: Use vcpu_idx, not vcpu_id, for GA log tag/metadata

Use a vCPU's index, not its ID, for the GA log tag/metadata that's used to
find and kick vCPUs when a device posted interrupt serves as a wake event.
Lookups on a vCPU index are O(fast) (not sure what xa_load() actually
provides), whereas a vCPU ID lookup is O(n) if a vCPU's ID doesn't match
its index.

Unlike the Physical APIC Table, which is accessed by hardware when
virtualizing IPIs, hardware doesn't consume the GA tag, i.e. KVM _must_
use APIC IDs to fill the Physical APIC Table, but KVM has free rein over
the format/meaning of the GA tag.

Tested-by: Sairaj Kodilkar <sarunkod@amd.com>
Link: https://lore.kernel.org/r/20250611224604.313496-57-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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