hw/arm/virt: don't use a15memmap directly

We should always go through VirtBoardInfo when we need the memmap.
To avoid using a15memmap directly, in this case, we need to defer
the max-cpus check from class init time to instance init time. In
class init we now use MAX_CPUMASK_BITS for max_cpus initialization,
which is the maximum QEMU supports, and also, incidentally, the
maximum KVM/gicv3 currently supports. Also, a nice side-effect of
delaying the max-cpus check is that we now get more appropriate
error messages for gicv2 machines that try to configure more than
123 cpus. Before this patch it would complain that the requested
number of cpus was greater than 123, but for gicv2 configs, it
should complain that the number is greater than 8.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-id: 1445189728-860-3-git-send-email-drjones@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
