bhyve: rename 'user_dev' with 'devices'

Bhyve don't use 'user' specifier for emulated devices. And
using 'user' adds duality.

Reviewed by: corvink, rew
MFC after: 1 week
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D40106

(cherry picked from commit b10d65a420f068bccf6aaf2942157a5347211dc5)