xmlrpc: Use correct base class for parsing request

Previously we always used SecureXMLRPCServer.parse_request, even when
using SimpleXMLRPCServer. That happened to work since both inherit
parse_request from BaseHTTPServer. This patch use the correct base
class, so if one of them will override parse_request, we use the correct
method.

Change-Id: I4ef737c02afb97580179f381e0f227ee94e1383f
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24289
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Liron Ar <laravot@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>