x86: fixup IRQs when CPUs go down during shutdown

x86: fixup IRQs when CPUs go down during shutdown

Commit fc0c3fa2ad5c ("x86/IO-APIC: fix setup of Xen internally used IRQs
(take 2)") introduced a regression on some hardware where Xen would hang
during shutdown, repeating the following message:
APIC error on CPU0: 08(08), Receive accept error

This appears to be because an interrupt (in this case from the serial
console) destined for a CPU other than the boot CPU is left unhandled so
an APIC error on CPU 0 is generated instead.

To fix this, before taking down the non-boot CPUs, call fixup_irqs()
with a CPU mask of only the boot CPU to reset the IRQ affinities
correctly.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>