Mark CPU present when it is detected

Mark CPU present when it is detected

Currently a CPU is marked as present only after it has been kicked off
successfully, i.e. before the CPU is brought up, it is not
present. This patch try to mark CPU as present when it is detected
(either through MPS table or ACPI). If it can't be brought up
successfully, it will be marked as non-present again.  This change is
mainly for CPU hot-plug. As discussed, we'd take two step for physical
CPU hot-add. A CPU is firstly marked as present, and later will bring
as online.

Also, In smp_boot_cpus(), xen need only scan all present CPU, and no
need to loop from 0... NR_CPUS. With this change, the bios_cpu_apicid
is not needed anymore.

Signed-off-by: Jiang, Yunhong <yunhong.jiang@intel.com>