[ARM] Increase default max VCPU and Guest counts

We update defconfigs of archs to increase CONFIG_MAX_VCPU_COUNT
to 64 and CONFIG_MAX_GUEST_COUNT to 16. This means by default
we can have maximum 16 Guests and maximum of 64 VCPUs (normal +
orphan) on any host. Users can still change this limits using
menuconfig.

Signed-off-by: Anup Patel <anup@brainfault.org>