bhyve: create all vcpus on startup

vcpus could be restarted by the guest by sending an INIT SIPI SIPI
sequence to a vcpu. That's not supported by bhyve yet but it will be
supported in a future commit. So, create the vcpu threads only once on
startup to make restarting a vcpu easier.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D35621
Sponsored by: Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 9cc9abf409cc9d761f42b16b0dc8f5c0cc4f9a44)