hw/arm/virt: fix max-cpus check

mach-virt doesn't yet support hotplug, but command lines specifying
-smp <num>,maxcpus=<bigger-num> don't fail. Of course specifying
bigger-num as something bigger than the machine supports, e.g. > 8
on a gicv2 machine, should fail though. This fix also makes mach-
virt's max-cpus check truly consistent with the one in vl.c:main,
as the one there was already correctly checking max-cpus instead
of smp-cpus.

Reported-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1454511578-24863-1-git-send-email-drjones@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
