linux-user/main: support dfilter

This adds the -dfilter support to linux-user. There is a minor
checkpatch complaint about formatting which I've ignored for aesthetic
reasons.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
