xen/x86: Call cpu_startup_entry(CPUHP_AP_ONLINE_IDLE) from xen_play_dead()

This call has always been missing from xen_play dead() but until
recently this was rather benign. With new cpu hotplug framework
(commit 8df3e07e

 ("cpu/hotplug: Let upcoming cpu bring itself fully up").
however this call is required, otherwise a hot-plugged CPU will not
be properly brough up (by never calling cpuhp_online_idle())

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>