When trying to guarantee a minimum CPU percentage of CPU for FreeBSD itself (from a few VMs), I found that though FreeBSD and latest vm-bhyve have the capability to do this, it's not enabled by default.
The following steps described here worked for me in FreeBSD 11.1 / vm-bhyve 1.1.8:
kern.racct.enable=1in loader.conf and reboot
rctl. You can either use options provided in vm-bhyve, or call rctl manually.