xen/smp: Use smp_store_boot_cpu_info() to store cpu info for BSP during boot time.

Git commit 30106c17
("x86, hotplug: Support functions for CPU0 online/offline") alters what
the call to smp_store_cpu_info() does. For BSP we should use the
smp_store_boot_cpu_info() and for secondary CPU's the old
variant of smp_store_cpu_info() should be used. This fixes
the regression introduced by said commit.

Reported-and-Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>