Make ich9_lpc_update_pic take care only of GSIs 0-15, and
ich9_lpc_update_apic take care only of GSIs 16-23. Assert
that they are called with the correct GSI indices.
Reviewed-by: Efimov Vasily <real@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>