xen/acpi: xen cpu hotplug minor updates

Recently at native Rafael did some cleanup for acpi, say, drop
acpi_bus_add, remove unnecessary argument of acpi_bus_scan,
and run acpi_bus_scan under acpi_scan_lock.

This patch does similar cleanup for xen cpu hotplug, removing
redundant logic, and adding lock.

Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>