linux-user: Add loop control ioctls

Add support for the /dev/loop-control ioctls:
 LOOP_CTL_ADD
 LOOP_CTL_REMOVE
 LOOP_CTL_GET_FREE

[RV: fixed to apply to new header guards]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>