xen/arm: don't try to re-register vcpu_info on cpu_hotplug.

Call disable_percpu_irq on CPU_DYING and enable_percpu_irq when the cpu
is coming up.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>