bhyve(8) sports the -D flag, which causes it to destroy the VM context
before exiting. The only interface currently available to do this is a
sysctl, which doesn't work since bhyve(8) runs in capability mode, so -D
is just broken.
I think the right solution is to move to a fd-based interface for
creating and destroying VM contexts, but in the meantime it seems
reasonable to poke a hole in the sandbox to unbreak the option. A
casper service just to destroy the VM is a bit silly.