bhyve: Disable thread safety analysis

The warnings that arise are bogus and have to be muted with
__no_lock_analysis in most cases. As a step towards enabling the
default warning level for bhyve, just disable them.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D37295

(cherry picked from commit 1a8e52391be837bb2851065b4bad6c163db9edef)