bhyvectl: Address compiler warnings and bump WARNS

Avoid unaligned accesses in cpu_vendor_intel() and address a few other
nits. No functional change intended.

Reviewed by: corvink, rew, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38839

(cherry picked from commit 3f5d875a27318a909f23a2b7463c4b2d963085df)