Move iothreadspin information into iothreadids

Remove the iothreadspin array from cputune and replace with a cpumask
to be stored in the iothreadids list.

Adjust the test output because our printing goes in order of the iothreadids
list now.

tests/qemuxml2xmloutdata/qemuxml2xmlout-cputune-iothreads.xml

@@ -12,8 +12,8 @@
     <vcpupin vcpu='1' cpuset='1'/>
     <vcpupin vcpu='0' cpuset='0'/>
     <emulatorpin cpuset='1'/>
-    <iothreadpin iothread='2' cpuset='3'/>
     <iothreadpin iothread='1' cpuset='2'/>
+    <iothreadpin iothread='2' cpuset='3'/>
   </cputune>
   <os>
     <type arch='i686' machine='pc'>hvm</type>