vl: fix max_cpus check

We should confirm max_cpus, which is >= smp_cpus, is
<= the machine's true max_cpus, not just smp_cpus.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
