This function pointer is used to call a system-specific
function for disabling the IO-APIC. Currently this is used
for IRQ remapping which has its own disable routine.

Also introduce the necessary infrastructure in the interrupt
remapping code to overwrite this and other function pointers
as necessary by interrupt remapping.
Signed-off-by: NJoerg Roedel <joro@8bytes.org>
Acked-by: NSebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>