qemu: support configuring usb3 controller port count

This adds a ports= attribute to usb controller XML, like

  <controller type='usb' model='nec-xhci' ports='8'/>

This maps to:

  qemu -device nec-usb-xhci,p2=8,p3=8

Meaning, 8 ports that support both usb2 and usb3 devices. Gerd
suggested to just expose them as one knob.

https://bugzilla.redhat.com/show_bug.cgi?id=1271408

docs/schemas/domaincommon.rng

@@ -1743,7 +1743,8 @@
               </attribute>
             </optional>
           </group>
-          <!-- usb has an optional attribute "model", and optional subelement "master" -->
+          <!-- usb has an optional attribute "model",
+               and optional subelements "master" and "ports" -->
           <group>
             <attribute name="type">
               <value>usb</value>
@@ -1768,6 +1769,11 @@
             <optional>
               <ref name="usbmaster"/>
             </optional>
+            <optional>
+              <attribute name="ports">
+                <ref name="unsignedInt"/>
+              </attribute>
+            </optional>
           </group>
           <!-- pci has an optional attribute "model" -->
           <group>