fstab-generator: use log message that matches reality

fstab-generator: use log message that matches reality

We *assume* that when /sys is read-only, we're running in a container. But
there can other reasons, for example root is mount ro and nobody has mounted
/sys yet, or somebody forgot to add /sys to the list of filesystem not to
remount ro in a sandbox. So let's actually say what we know instead of assuming.

systemd-fstab-generator was reporting that it's running in a container and I
spent a good few minutes trying to figure out why 'systemd-detect-virt -c'
disagrees, before noticing that it's just checking a different condition.