qemu: Implement CPUs check against machine type's cpu-max

Implement check whether (maximum) vCPUs doesn't exceed machine
type's cpu-max settings.

On older versions of QEMU the check is disabled.

Signed-off-by: Michal Novotny <minovotn@redhat.com>

docs/schemas/capability.rng

@@ -290,6 +290,11 @@
           <text/>
         </attribute>
       </optional>
+      <optional>
+        <attribute name='maxCpus'>
+          <ref name='unsignedInt'/>
+        </attribute>
+      </optional>
       <text/>
     </element>
   </define>