kern: Introduce RLIMIT_VMM

This change introduces a new per-UID limit for controlling the
number of vmm instances, in anticipation of unprivileged bhyve.
This allows ut to limit the amount of kernel memory allocated
by the vmm driver and prevent potential memory exhaustion attacks.

Differential Revision: https://reviews.freebsd.org/D53728
Reviewed by: markj, olce, corvink
MFC after: 3 months
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.