hotplug: Prevent alloc/free of irq descriptors during cpu up/down (again)

Now that Xen no longer allocates irqs in _cpu_up() we can restore
commit a8994181 ("hotplug: Prevent alloc/free of irq descriptors
during cpu up/down")

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
CC: x86@kernel.org
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>