systemd: Use the default log target ("journal-or-kmsg")

This ensures that early systemd messages end up in the journal (via
the kmsg buffer).
test-instrumentation.nix: Improve logging

In particular, don't clobber the serial console with duplicate output
from systemd and the journal, and increase the log level.
test-instrumentation.nix: Set an empty root password

This makes it easier to log in during interactive sessions.