x86/xen: put setup.c, pmu.c and apic.c under CONFIG_XEN_PV

xen_pmu_init/finish() functions are used in suspend.c and
enlighten.c, add stubs for now.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>