vmm: Add ability to destroy VMs on close

This change adds the ability to tie a virtual machine's lifecycle to
a /dev/vmmctl file descriptor. A user can request vmmctl to destroy a
virtual machine on close using the VMMCTL_CREATE_DESTROY_ON_CLOSE flag
when creating the virtual machine. vmmctl tracks such virtual machines
in per-descriptor lists.

Differential Revision: https://reviews.freebsd.org/D53729
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.
MFC after: 3 months