tools/python: Replace python string exceptions with ValueError exceptions

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>