bhyveload(8): document some SECURITY CONSIDERATIONS

The situation is improved now that we're running in a sandbox, but there
is still some host machine access that could be concerning depending on
the context. These concerns may be somewhat mitigated by the fact that
the host machine usually provides the loader binary, even when the guest
image is providing the loader scripts -- they only bring the lua
scripts, and they have to be able to execute arbitrary syscalls rather
than the interfaces provided by libsa(3).

Reviewed by: jhb, markj
Differential Revision: https://reviews.freebsd.org/D43299