ports/extra/virtualbox

  Upgrading a kernel should not affect external modules. Modules which are
  included in the kernel are installed in: /lib/modules/$(KERNELRELEASE)/kernel
  And external modules are installed in: /lib/modules/$(KERNELRELEASE)/extra

  Anyway if you want to compile and install virtualbox modules again you have
  some alternatives:
  # make -C /opt/VirtualBox/src/vboxhost \
    MODULE_DIR=/lib/modules/$(uname -r)/extra install

  Or if you want to an specific kernel version (for example: 4.1.8)
  # make -C /opt/VirtualBox/src/vboxhost \
    KERN_DIR=/usr/src/linux-4.1.8 \
    MODULE_DIR=/lib/modules/4.1.8/extra install

  For modules load on boot add this into MODULES array in /etc/rc.conf
    vboxdrv
    vboxnetadp
    vboxnetflt
    vboxpci