Devices that use ISA IRQs expect them to be already configured, and don't
call bus_config_intr, which prevents those IRQs from working on Xen. In
order to solve it pre-register all the legacy IRQs with the default values
(edge triggered, low polarity) if no override is found.
While there add a panic if the registration of an interrupt override fails.
Sponsored by: Citrix Systems R&D
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