percpu: use correct percpu sect size to support 64 vcpus

percpu had too little space allocated to support 64 vcpus, which
lead to a crash when booting with more than 13 vcpus. Fix it by
using a correct size to support 64 vcpus.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>