bhyve: change suspend exit code

Currently, after suspending, bhyve(8) exits with 0. This code is
also used to indicate that a VM was rebooted. To differentiate
reboot and suspend, use the next available exit code, 5, for suspend.

Approved by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D54087