Stop running auxiliary programs in systemd scopes

Stop running auxiliary programs in systemd scopes

Similar to the same change made in systemd-vmspawn, let's stop running
virtiofsd, systemd-journal-remote and swtpm in scopes. Nobody ever makes
use of the features this provides and it simplifies our code quite a bit.

This also means we drop the UnitProperties setting, which was effectively
unused anyway.

This allows us to get rid of the --suspend setting in mkosi-sandbox, which
only really existed to allow waiting for systemd-run to finish its setup
before registering the machine. Because registering a machine means it needs
a cgroup, we allow systemd-machined to create the scope itself if needed.