Allow a privileged Xen guest (Dom0) to parse the MADT ACPI interrupt
overrides and register them with the interrupt subsystem.
Also add a Xen specific implementation for bus_config_intr that
registers interrupts on demand for all the vectors less than
FIRST_MSI_INT.
Sponsored by: Citrix Systems R&D
x86/xen/pvcpu_enum.c:
- Use helper functions from x86/acpica/madt.c in order to parse
interrupt overrides from the MADT.
- Walk the MADT and register any interrupt override with the
interrupt subsystem.
x86/xen/xen_nexus.c:
- Add a custom bus_config_intr method for Xen that intercepts calls
to configure unset interrupts and registers them on the fly (if the
vector is < FIRST_MSI_INT).
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