x86/xen: split xen_smp_prepare_boot_cpu()

Split xen_smp_prepare_boot_cpu() into xen_pv_smp_prepare_boot_cpu() and
xen_hvm_smp_prepare_boot_cpu() to support further splitting of smp.c.

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