xen/acpi: move xen_acpi_get_pxm under CONFIG_XEN_DOM0

To avoid compile issue and it's meanigfull only under CONFIG_XEN_DOM0.

In file included from linux/arch/x86/xen/enlighten.c:47:0:
	linux/include/xen/acpi.h:75:76: error: unknown type name ‘acpi_handle’
make[3]: *** [arch/x86/xen/enlighten.o] Error 1

Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
[v1: Fixed spelling mistakes]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>