adjust fstab devices to use labels, if possible
otherwise the system will not be able to automatically mount root
It'll crash to the mountroot dialog
pressing "?" will show a list of devices
depending on the device model set for the virtual disk you would end up with either of
ada0a, vbtd0a or da0a
(there might be a typo in vbtd0a, the system is currently off)
Enter your root device like this
And it'll boot!
Adjust your network device names
in /etc/rc.conf you'll need to adjust the name of your network device.
The old one will have been xn0, and the new one can be either em0 (emulated driver) or vn<something>0.
The latter is the higher-performing virtio device. It *felt* slower than xn0, but that is not really a test at all, just a feeling.
proper automatic shutdown is via acpid
so you need to have it enabled, and for me it only worked after an initial reboot, not on the first time
Different from Xen there seems to be no proper virtual clock driver
That means you'll get the horrible "microuptime went backwards" messages if you save/resume the vm, and possibly after live migrations, too.
This bears a high operational risk and, unless you find a way to fix it, you should not - I repeat - NOT run databases or other time-sensitive applications in FreeBSD on KVM.
At least test it doesn't happen on live migration, suspend/resume isn't the most common use case anyway.