Default to enabling the serial console in addition to the video console in /etc/loader.conf.
Change default speed 115200 (as opposed to 9600). Add an entry for /dev/ttyV0.0 (virtio / Linux KVM) "onifconsole" to enable serial console out-of-the-box for Linux KVM.
The comconsole
setting makes the kernel emit messages at boot (and while running).
A slightly less invasive option would be to have ttyV0.0 set to "onifexists" instead (enabling interactive logins, but no console message output).
Relevant reading:
- Handbook section 27.6.3: Setting a Faster Serial Port Speed
- man 8 loader
- FreeBSD D29873 (which adds a Xen-compatible virtual serial console entry)