Add knobs.

protogeni/test/xen-shared.rspec

 <?xml version="1.0" encoding="UTF-8"?>
 <rspec xmlns="http://www.protogeni.net/resources/rspec/2"
+       xmlns:emulab="http://www.protogeni.net/resources/rspec/ext/emulab/1"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:schemaLocation="http://www.protogeni.net/resources/rspec/2 http://www.protogeni.net/resources/rspec/2/request.xsd"
        type="request" >
@@ -10,6 +11,7 @@
   <node client_id="xen2"
         exclusive="false">
     <sliver_type name="emulab-xen">
+      <emulab:xen cores="1" ram="512"/>
       <disk_image
 	  name="urn:publicid:IDN+emulab.net+image+emulab-ops//FEDORA15-STD" />
     </sliver_type>
@@ -17,6 +19,7 @@
   <node client_id="xen3"
         exclusive="false">
     <sliver_type name="emulab-xen">
+      <emulab:xen cores="1" ram="512" disk="8"/>
       <disk_image
 	  name="urn:publicid:IDN+emulab.net+image+emulab-ops//FBSD82-STD" />
     </sliver_type>