Extend the max vcpu number for HVM guest.

Extend the max vcpu number for HVM guest.
 - Originally the max vcpu number for HVM guest is 32, this patch
 extend the number to 128 on x86_64 hypervisor. (For i386 hypervisor,
 the max vcpu number  is still 32).
 - This patch extends the mp-table size to fit more vcpus.
 - HVM PV driver should call VCPUOP_register_vcpu_info hypercall to
 initialize the vcpu info if the vcpu number is more than 32.

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>