Declarations of functions that convert between host and target structs.

Co-authored-by: Michal Meloun <mmel@FreeBSD.org>

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Karim Taha <kariem.taha2.7@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>