hw/pci-host/gpex: Improve INTX to gsi routing error checking

We exposed gpex_set_irq_num() for machines to set the INTx to
GSI routing. However if the machine forgets to call that
function we currently do not check the association was properly
done. Let's initialize gsi values to -1 and if this value is
found in gpex_route_intx_pin_to_irq, set the routing mode as
disabled.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Message-id: 1508776211-22175-1-git-send-email-eric.auger@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
