hw/isa: Factor isa_bus_get_irq() out of isa_get_irq()

hw/isa: Factor isa_bus_get_irq() out of isa_get_irq()

isa_get_irq() was added in commit 3a38d437ca
("Add isa_reserve_irq()" Fri Aug 14 11:36:15 2009) as:

    a temporary interface to be used to allocate ISA IRQs for
    devices which have not yet been converted to qdev, and for
    special cases which are not suited for qdev conversions,
    such as the 'ferr'.

We still use it 14 years later, using the global 'isabus'
singleton. In order to get rid of such *temporary* interface,
extract isa_bus_get_irq() which can take any ISABus* object.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230215161641.32663-3-philmd@linaro.org>