xen/arm64: increase MAX_VIRT_CPUS to 128 on arm64

xen/arm64: increase MAX_VIRT_CPUS to 128 on arm64

After we have increased the size of GICR in address space for guest
and made use of both AFF0 and AFF1 in (v)MPIDR, we are now able to
support up to 4096 vCPUs in theory. However, it will cost 512M
address space for GICR region, which is unnecessarily big at the
moment. Considering the max CPU number that GIC-500 can support and
the old value of MAX_VIRT_CPUS before commit aa25a61, we increase
its value to 128.

Signed-off-by: Chen Baozi <baozich@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>