lib: fix the catch of AUM crash

AUM crash is detected as unexpected AUM restart. However due
races on startup the cursor for logs check may catch the expected
restart and treat it as unexpected.
Reset the log cursor to check events occurred during update only.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

update-manager-common.sh

@@ -121,6 +121,10 @@ apply_update_sync()
 	# and if the handler quits, AUM reboots. And if AUM reboots, LAVA timeouts.
 	nohup updatectl --register-upgrade-handler & UPGRADEHANDLERPID=$!
 
+	# Update the cursor to catch events after restart
+	CURSOR="$(journalctl --show-cursor -n 0 -o cat)"
+	CURSOR="${CURSOR#-- cursor: }"
+
 	# Applying update is asynchronous, updatectl spawns the update and returns immediately
 	if [ -n "$OTA" ]; then
 		apply_update_async