dbus-unit: always load the unit before handling a message for it

dbus-unit: always load the unit before handling a message for it

We need to be able to show the properties even of inactive units.
systemctl loads the unit before getting its properties, but this is racy
as the garbage collector may kick in right after the loading.

Fix it by always loading the unit before handling a message for it.

https://bugzilla.redhat.com/show_bug.cgi?id=814966#c6