tools/python: Replace python string exceptions with ValueError exceptions

There are at least some syntax errors when trying to use the xen utils with
python2.6.  The attached patch changes these string exception into
ValueErrors:

   - tools/python/xen/util/bugtool.py (getBugTitle)
   - tools/python/xen/xend (class XendBase): not catched
   - tools/python/xen/xm/xenapi_create.py (sxp2xmlconvert_sxp_to_xml):
     the method already raises a ValueError for similiar condition.
   - tools/python/xen/xm/main.py (xm_network_attach): not catched.

For all but maybe the first one, the replacement of the string
exceptions into ValueErrors seems to be safe.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/python/xen/util/bugtool.py

@@ -141,7 +141,7 @@ def getBugTitle(bug):
     finally:
         f.close()
 
-    raise "Could not find title of bug %d!" % bug
+    raise ValueError("Could not find title of bug %d!" % bug)
 
 
 def send(bug, conn, fd, filename, username, password):

tools/python/xen/xend/XendBase.py

@@ -115,7 +115,7 @@ class XendBase:
             # In OSS, ref == uuid
             return uuid
         else:
-            raise "Big Error.. TODO!"
+            raise ValueError("Big Error.. TODO!")
 
     def get_all_records(cls):
         return dict([(inst.get_uuid(), inst.get_record())

tools/python/xen/xm/main.py

@@ -2796,7 +2796,7 @@ def xm_network_attach(args):
                              for ref, record in server.xenapi.network
                              .get_all_records().items()])
             if bridge not in networks.keys():
-                raise "Unknown bridge name!"
+                raise ValueError("Unknown bridge name!")
             return networks[bridge]
 
         vif_conv = {

tools/python/xen/xm/xenapi_create.py

@@ -707,7 +707,7 @@ class sxp2xml:
                 vm.attributes['security_label'] = \
                                     security.set_security_label(sec_data[0][1][1],sec_data[0][2][1])
             except Exception, e:
-                raise "Invalid security data format: %s" % str(sec_data)
+                raise ValueError("Invalid security data format: %s" % str(sec_data))
 
         # Make the name tag