xen/x86: Move irq allocation from Xen smp_op.cpu_up()

Commit ce0d3c0a ("genirq: Revert sparse irq locking around
__cpu_up() and move it to x86 for now") reverted irq locking
introduced by commit a8994181 ("hotplug: Prevent alloc/free
of irq descriptors during cpu up/down") because of Xen allocating
irqs in both of its cpu_up ops.

We can move those allocations into CPU notifiers so that original
patch can be reinstated.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>