vmm: Start using exterror

For now, just describe the error where an unprivileged user attempts to
run a VM without DESTROY_ON_CLOSE semantics, i.e., monitor mode.

Reviewed by: bnovkov
MFC after: 2 months
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54743