iommu/amd: WARN if KVM attempts to set vCPU affinity without posted intrrupts

WARN if KVM attempts to set vCPU affinity when posted interrupts aren't
enabled, as KVM shouldn't try to enable posting when they're unsupported,
and the IOMMU driver darn well should only advertise posting support when
AMD_IOMMU_GUEST_IR_VAPIC() is true.

Note, KVM consumes is_guest_mode only on success.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250404193923.1413163-7-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>