x86/xen: split off smp_hvm.c

Move PVHVM related code to smp_hvm.c. Drop 'static' qualifier from
xen_smp_send_reschedule(), xen_smp_send_call_function_ipi(),
xen_smp_send_call_function_single_ipi(), these functions will be moved to
common smp code when smp_pv.c is split.

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