[bug] Unsubscribe requests are not sent when MSU exits

when MSU exits, send first the requests to unsubscribe
for device contexts notification, and delay the g_main_loop exit
for 1 second to give them a chance to be really sent.

Fix issue: https://github.com/01org/media-service-upnp/issues/187

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>