Hide errors from qubes.WaitForSession

Especially "Killed" message, which is normal as 'sleep' is signalled with
SIGKILL once session is ready.

qubes-rpc/qubes.WaitForSession

@@ -1,2 +1,2 @@
 read USERNAME
-su -c 'echo $$ >> /tmp/qubes-session-waiter; [ ! -f /tmp/qubes-session-env ] && exec sleep 365d' - "$USERNAME"
+su -c 'echo $$ >> /tmp/qubes-session-waiter; [ ! -f /tmp/qubes-session-env ] && exec sleep 365d' - "$USERNAME" 2> /dev/null