hw/pci: Extract pci_bus_change_irq_level() from pci_change_irq_level()

Extract pci_bus_change_irq_level() from pci_change_irq_level() to
make it clearer it operates on the bus.

Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201024203900.3619498-2-f4bug@amsat.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>