nixos/testing: Fix output of systemd messages

Regression introduced by d4468bed.

No systemd messages are shown anymore during VM test runs, which is not
very helpful if you want to find out about failures.

There is a bit of a conflict between testing and the change that
introduced the regression. While the mentioned commit makes sure that
the primary console is tty0 for virtualisation.graphics = false, our VM
tests need to have the serial console as primary console.

So in order to support both, I added a new virtualisation.qemu.consoles
option, which allows to specify those options using the module system.

The default of this option is to use the changes that were introduced
and in test-instrumentation.nix we use only the serial console the same
way as before.

For test-instrumentation.nix I didn't add a baudrate to the serial
console because I can't find a reason on top of my head why it should
need it. There also wasn't a reason stated when that was introduced in
7499e4a5

.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @flokli, @dezgeg, @edolstra