Since its introduction in commit ac1970fb, address_space_rw()
takes a boolean 'is_write' argument. Fix the codebase by using
an explicit boolean type.

This commit was produced with the included Coccinelle script
scripts/coccinelle/exec_rw_const.

Inspired-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>