Add comment which describes the exit status codes returned from
/usr/sbin/bhyve.  These are in src/usr.sbin/bhyve/bhyverun.c.

Reviewed by: neel


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270512 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f