machined: only Unref units that we AddRef'd

machined: only Unref units that we AddRef'd

b92d0b4c5adef37e9de8f6cc22a0e27b97fcf3ad added AddRef to the StartTransientUnit
call in machine_start_scope()/manager_start_scope() and a corresponding Unref
call in machine_stop_scope(). But when we are running systemd-nspawn@ with
--keep unit, the unit is not created by machined so the AddRef never happens.
Then when trying to stop the unit, we'd get:

systemd-machined[1101]: Sent message type=method_call sender=n/a destination=org.freedesktop.systemd1 path=/org/freedesktop/systemd1 interface=org.freedesktop.systemd1.Manager member=UnrefUnit cookie=37 reply_cookie=0 signature=s error-name=n/a error-message=n/a
systemd-machined[1101]: Got message type=error sender=:1.1 destination=:1.13 path=n/a interface=n/a member=n/a cookie=2443 reply_cookie=37 signature=s error-name=org.freedesktop.systemd1.NotReferenced error-message=Unit has not been referenced yet.
systemd-machined[1101]: Failed to drop reference to machine scope, ignoring: Unit has not been referenced yet.