utils: Ensure that XMLRPC threads do not delay shutdown

The threads created by the XMLRPC server are not daemon threads, so they
may delay vdsm shutdown if a long request happen to run during shutdown.
This patch configure XMLRPC server to use daemon threads.

The daemon_threads class attribute is used by
SocketServer.ThreadingMixIn. When defined in base class, both the simple
and secure servers get this configuration through inheritance.

This change is required for adding HTTP/1.1 support, where XMLRPC server
threads become long living threads.

Change-Id: I67d3a1b062e7de61e7878b47291cca946f0f13af
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24555
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Adam Litke <alitke@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>