xen/cpuhotplug: Replace cpu_up/down() with device_online/offline()

The core device API performs extra housekeeping bits that are missing
from directly calling cpu_up/down().

See commit a6717c01 ("powerpc/rtas: use device model APIs and
serialization during LPM") for an example description of what might go
wrong.

This also prepares to make cpu_up/down() a private interface of the cpu
subsystem.

Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lkml.kernel.org/r/20200323135110.30522-14-qais.yousef@arm.com