bhyve: allow for automatic destruction on power-off

Introduce -D flag that allows for the VM to be destroyed on guest initiated
power-off by the bhyve(8) process itself.
This is quality of life change that allows for simpler deployments without
the need for bhyvectl --destroy.

Requested by: swills
Reviewed by: 0mp (manpages), grehan, kib, swills
Approved by: kib (mentor)
MFC after: 2 weeks
Sponsored by: Mysterious Code Ltd.
Differential Revision: https://reviews.freebsd.org/D25414