bhyve: Address warnings about potential unaligned accesses in fwctl.c

This silences some warning about potential unaligned accesses. No
functional change intended.

MFC after: 1 week
Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D37288

(cherry picked from commit f64f34380925a4ddaf1c140c35f46408b74110ac)