bhyve: Enable the default compiler warnings

Disable -Wcast-align for now since we have many instances of that
warning (I fixed some but not most of them) and platforms on which bhyve
runs don't particularly care about unaligned accesses.

Reviewed by: corvink
Differential Revision: https://reviews.freebsd.org/D37296

(cherry picked from commit 71ebd117386cda6410ca65eb487b63e5dedf3245)