Even though there is only one monitor, and thus no race on this
global data object, there is also no point in having it.  We can
just as well record the decision in the read_memory_function that
we select.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>