CA-102285: Use the stream API to read the XML RRD file.

Imported from xen-api.git f35f24e8dd7bf54a2db6b788316335b8c8111b82

Signed-off-by: Jerome Maloberti <jerome.maloberti@citrix.com>
Imported-by: John Else <john.else@citrix.com>
Merge pull request #3 from johnelse/cp-5129

CA-102285: Use the stream API to read the XML RRD file.