hw/misc/unimp: Move struct to header file

hw/misc/unimp: Move struct to header file

Move the definition of the struct for the unimplemented-device
from unimp.c to unimp.h, so that users can embed the struct
in their own device structs if they prefer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180220180325.29818-10-peter.maydell@linaro.org