[XEND] Updates to SR and VDI implementations

* Moved xenapi transport util method, stringify to xen.util.xmlrpclib2
* XenVDI now preserves configuration to an XML-ish file
* Update Xen API's class names to be all lowercase
* Update get_by_label to get_by_name_label and return sets as the API
  expects.
* Add support for VBD creation with a VDI reference.

Signed-off-by: Alastair Tse <atse@xensource.com>

tools/python/scripts/xapi.vdicfg.py

@@ -0,0 +1,7 @@
+name_label = 'VDI 1'
+name_description = ''
+virtual_size = 10 * 1024
+sector_size = 1024
+type = 'system'
+sharable = False
+read_only = False