numad: Always output 'placement' of <vcpu>

<vcpu> is not an optional node. The value for its 'placement'
actually always defaults to 'static' in the underlying codes.
(Even no 'cpuset' and 'placement' is specified, the domain
process will be pinned to all the available pCPUs).

tests/qemuxml2argvdata/qemuxml2argv-disk-floppy-tray-no-device-cap.xml

@@ -3,7 +3,7 @@
   <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
   <memory>219136</memory>
   <currentMemory>219136</currentMemory>
-  <vcpu>1</vcpu>
+  <vcpu placement='static'>1</vcpu>
   <os>
     <type arch='i686' machine='pc'>hvm</type>
     <boot dev='hd'/>