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

This ensures that early systemd messages end up in the journal (via
the kmsg buffer).

(cherry picked from commit 0d3b3bd01bb4dc89fef477920afaf3c8e56dfde6)
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.

(cherry picked from commit eab25b104ad12b75a8b9a41ce3ac8b3b931144ef)
test-instrumentation.nix: Set an empty root password

This makes it easier to log in during interactive sessions.

(cherry picked from commit 2b9ea7fd906b759115b76d7e0f08b92f1b316dbf)