conf: Adjust the iothreadsched expectations

With iothreadid's allowing any 'id' value for an iothread_id, the
iothreadsched code needs a slight adjustment to allow for "any"
unsigned int value in order to create the bitmap of ids that will
have scheduler adjustments. Adjusted the doc description as well.

tests/qemuxml2argvdata/qemuxml2argv-cputune-iothreadsched-toomuch.xml

@@ -13,7 +13,8 @@
     <vcpupin vcpu='1' cpuset='1'/>
     <emulatorpin cpuset='1'/>
     <vcpusched vcpus='0-1' scheduler='fifo' priority='1'/>
-    <iothreadsched iothreads='2' scheduler='batch'/>
+    <iothreadsched iothreads='1' scheduler='batch'/>
+    <iothreadsched iothreads='2' scheduler='fifo' priority='1'/>
   </cputune>
   <os>
     <type arch='i686' machine='pc'>hvm</type>