emulators/virtualbox-ose-kmod: Fix hang at module loading time

After src commit e266a0f7f001 a module is not allowed to call
kern_kldload while being loaded. The virtualbox module is doing
exactly this though. virutalbox used to do this because the ng_ether
netgraph node did not provide version information in the past and
there was no way to properly depend on it.

ng_ether has gained versioning information in base r238844 more
than ten years ago, so we can now unconditionally properly depend
on it, since this revision is now included in all supported FreeBSD
versions.

Thanks to Tomoaki AOKI for identifying the base commuit exposing
the issue.
PR: 256505
Tested by: Tomoaki AOKI <junchoon@dec.sakura.ne.jp>
MFH: 2021Q2
Differential Revision: https://reviews.freebsd.org/D30722