[xenserver] added unplug_pbd request

lib/fog/xenserver/requests/compute/unplug_pbd.rb

@@ -0,0 +1,25 @@
+module Fog
+  module Compute
+    class XenServer
+
+      class Real
+
+        def unplug_pbd( ref )
+          @connection.request(
+            {:parser => Fog::Parsers::XenServer::Base.new, :method => 'PBD.unplug'}, 
+            ref
+          )
+        end
+
+      end
+
+      class Mock
+
+        def unplug_pbd( ref )
+          Fog::Mock.not_implemented
+        end
+
+      end
+    end
+  end
+end