xen, cpu_hotplug: call device_offline instead of cpu_down

xen, cpu_hotplug: call device_offline instead of cpu_down

When offlining a cpu, instead of cpu_down, call device_offline, which
also takes care of updating the cpu.dev.offline field. This keeps the
sysfs file /sys/devices/system/cpu/cpuN/online, up to date.  Also move
the call to disable_hotplug_cpu, because it makes more sense to have it
there.

We don't call device_online at cpu-hotplug time, because that would
immediately take the cpu online, while we want to retain the current
behaviour: the user needs to explicitly enable the cpu after it has
been hotplugged.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: konrad.wilk@oracle.com
CC: boris.ostrovsky@oracle.com
CC: david.vrabel@citrix.com
(cherry picked from commit 1c7a62137bb23bc8a2c05d1dad6105afa569b20e)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>