When I start a VM on VirtualBox 5.2.18 on my laptop, it reboots immediately. I do not even get a kernel panic message, just a hard reset. I do not have this issue with a Ubuntu 18.04 LiveUSB.
* FreeBSD 12.0-ALPHA2 r337934
* HP EliteBook 1040 G3
* Intel Core i7-6600U
* 16GB DDR4-2133
* 256GB M.2 SATA
* Desktop: graphics/drm-next-kmod and Gnome 3.18
Possibly the same issue as PR 230460
You may want to look at PR 220003 and experiment with your VM's network interface configuration (if it makes sense in your use case). It may be the same issue.