emulators/virtualbox-kmod: turn on VIMAGE by default for FreeBSD 12+

Hello,

With this change:
https://svnweb.freebsd.org/base?view=revision&revision=324810 I think
should also set VIMAGE options by default in emulators/virtualbox-kmod port
for FreeBSD 12+ via

.if ${OPSYS} == FreeBSD && ${OSVERSION} >= 1200051

or by separated meta port ( FLAVOR can help?) :

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215336

Without this change we will get panic on FreeBSD 12 + virtualbox-kmod on
boot.