host-utils: Fix coding style and add comments

Add function comments to the routines, documenting the corner
cases upon which we are standardizing.  Fix the few instances
of non-standard coding style.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>